A one-time password (OTP) only protects an account if it reaches the right person while it is still valid. SMS, USSD and voice calls are three ways to get that code to a phone, and each one trades reach, security and effort for your users differently.

This guide compares SMS vs USSD vs voice OTP so you can match the channel to the people you serve. The right choice also depends on what your OTP provider supports in your users’ country, so check that before you build.

Each OTP is tied to one sign-in or transaction and expires after a short window, often a few minutes, so an old code cannot be reused. Our guide on how long an OTP should stay valid covers how to set it.

SMS vs USSD vs voice OTP at a glance

A comparison of SMS, USSD, and Voice SMS for OTP delivery

SMS (Short Message Service) for OTP delivery

SMS OTP sends the code as a text message to the user’s phone. It needs nothing beyond the messaging app already on the handset. It works like this:

1. The user signs in or starts a transaction, and your application requests an OTP.
2. The provider sends the code by SMS. It stays valid for a set window, and if it expires, the user requests a new one.
3. The user types the code in, your application checks it, and access is granted.

Advantages of SMS for OTP delivery

• Works on any phone that receives texts: smartphones and feature phones alike, with no app to download.
• Works without internet: the code travels over the mobile network, so it reaches users with a weak or no data connection.
• Fast when the network is healthy: a code can arrive within seconds of the request.

Limitations of SMS for OTP delivery

• Network dependency: congestion or weak coverage can hold a message back until the code has expired.
• Phone-number risk: SMS OTP is exposed to SIM swap fraud and number porting, where an attacker takes control of the user’s number and receives the code. NIST’s digital identity guidelines advise checking for signs such as a SIM change, number porting or a device swap before sending a code over the phone network.
• Cost varies: what you pay per message depends on your provider and the destination country.

Best for

SMS OTP is best for general sign-in and transaction checks where mobile networks are reliable.

USSD for OTP delivery

Is USSD an OTP delivery channel? Only where your provider builds it. Unstructured Supplementary Service Data (USSD) is the menu system a user reaches by dialling a short code ending in the hash key, and when it carries an OTP, the code appears on screen during a session the user starts rather than arriving as a message.

Whether this option exists for your users depends on your provider and their country, so confirm it before you plan around it. When it is offered, it works like this:

• The user dials the service’s USSD code to start a session.
• The OTP appears on the screen during that session. It is not saved to an inbox, and it is gone when the session ends.

Advantages of USSD for OTP delivery

• No inbox copy: the code is not stored as a message, so nobody can scroll back to it later on the phone.
• Works on feature phones: any handset that can dial a USSD code can use it.
• No internet needed: the session runs over the mobile network, not mobile data.

Limitations of USSD for OTP delivery

• Depends on your provider: you cannot assume your provider offers USSD for OTP, and coverage varies by country.
• Possible charges for users: depending on the network, users may be charged for USSD sessions, and those charges add up for frequent users.
• Session dependency: the user must finish before the session times out. If they are too slow, they start again.
• The user does the work: they have to start the session themselves, which adds a step compared with a code that simply arrives.
• Network constraints: a poor connection can drop the session, which frustrates users mid-sign-in.

Best for

USSD OTP suits users on feature phones in markets where your provider offers it, as an option alongside SMS rather than a replacement for it.

Voice SMS for OTP delivery

Which authentication method uses voice communication to deliver codes? Voice OTP. Voice SMS, also called voice OTP, is a code read out on an automated call: the phone rings, a voice reads the digits aloud, and the user types them in.

• When a user starts a sign-in or transaction, your application requests an OTP.
• The user receives an automated call that reads the code aloud.
• The user listens, types the code into your application or website, and completes the check.

Advantages of Voice SMS for OTP delivery

• Accessible to users who find reading difficult: a spoken code works for people who cannot easily read a text message. In Ghana, the 2021 Population and Housing Census put the literacy rate of people aged 6 and older at 69.8%, while 55.2% of the rural population in that age group was literate in at least one language, against 80.6% of the urban population. If you serve users outside Ghana’s cities, a spoken code is worth offering.
• Sidesteps text-message problems: if the call connects, the code does not depend on SMS delivery.

Limitations of Voice SMS for OTP delivery

• Same phone-number risk as SMS: a call goes to whoever controls the number, so SIM swap and number porting affect voice exactly as they affect SMS. NIST’s current guidelines treat a one-time code sent over the phone network, whether by SMS or voice, as a restricted authenticator that an organisation may use only after assessing and accepting its risks.
• Privacy in public: hearing a code aloud in a crowded place can expose it to people nearby.
• Missed calls: the code only arrives if the user answers and hears it, which can delay time-sensitive transactions.
• Availability and cost vary: providers offer voice OTP in different countries and charge for it differently, so check both for your users.

Best for

Voice SMS suits users who are uncomfortable with written messages, and places where SMS delivery is less reliable than a phone call, provided your provider offers voice in that country.

Choosing the best method for OTP delivery

Choosing between OTP delivery methods comes down to who your users are and what each sign-in or transaction puts at risk. Work through the factors below.

Target audience

If your users have feature phones or live where internet access is limited, SMS and voice both work without data, and USSD is an option where your provider offers it. Voice helps users who find reading a code hard. For smartphone users on reliable networks, SMS covers most needs.

Which OTP delivery method is most secure?

SMS and voice OTP share the same weakness: both go to whoever controls the phone number. A USSD code that leaves no inbox copy is not, on that alone, stronger protection, so ask your provider how it secures the session.

Match the channel to the risk of the action. For high-value actions such as payments or account recovery, check for a recent SIM change before you send the code, and compare other options, including authenticator apps, in our guide to SMS OTP vs authenticator app vs email OTP.

Cost efficiency

Costs differ by provider, channel and destination country, so compare per-OTP rates for the countries your users are in rather than assuming one channel always costs less. Also check what you are billed for: every code sent, or only codes that are verified. You can see Arkesel’s current pricing for your market.

Reliability and speed

Consider how reliable mobile networks are where your users live. If SMS delivery is inconsistent, a voice resend, or USSD where your provider offers it, gives the user a second route to the code.

What should you do if an SMS OTP does not arrive?

Give the user a resend option after a short wait, and offer a second channel your provider supports for their country, such as a voice call. Check whether your provider switches channels for you or leaves the retry to your application. Our guide on what to do when an OTP SMS fails covers resend limits and cooldowns.

If your users are in Ghana, Nigeria, South Africa or Tanzania, Arkesel Phone Number Verification sends OTPs by SMS in all four countries and by voice call for Ghanaian numbers.

How Arkesel handles OTP delivery

Before you choose a provider, ask three questions: which channels it supports in each of your users’ countries, whether it switches channel for you when a message fails, and whether you pay per code sent or per verified code. Our comparison of OTP API providers covers what else to check. Here is how Arkesel answers them.

• You choose the channel on every request. Each call to the OTP API names SMS or voice, and there is no default.
• There is no automatic fallback. A failed SMS is not resent by voice for you, and nothing is sent over USSD. If you want a voice resend, your application makes a second request with voice as the channel. Our guide to integrating an SMS OTP API into your application walks through the build step by step.
• Voice OTP works for Ghanaian numbers only. Nigerian, South African and Tanzanian numbers receive OTPs by SMS only.
• Ghanaian users can view a missed SMS OTP by USSD. A Ghanaian user who was sent a code by SMS but did not receive it can dial a USSD code to view it. This is something the user does, not a delivery channel. It works for Ghanaian numbers only and is free for the user to dial. The current code is in the developer documentation, and our guide on how to let users view a missed SMS OTP by USSD shows how to add it to your verification screen.
• Arkesel generates the code; you set its length and expiry. Codes can be 6 to 15 digits long, with an expiry window you set between 1 and 10 minutes.
• You are billed per OTP sent, not per successful verification. See Arkesel pricing for your market.

Arkesel OTP channels by country

Arkesel Phone Number Verification is available in Ghana, Nigeria, South Africa and Tanzania, with channels scoped by country:

Bringing it all together: Choosing the best OTP solution

The right OTP delivery method, SMS, USSD or Voice SMS, depends on your users and on what your provider supports in their country. SMS offers wide reach, but shares its phone-number risk with voice. USSD works on feature phones where your provider offers it, but the user must start and finish a session.

Voice SMS reaches people of every literacy level, at the cost of privacy in public and unanswered calls. Letting users choose their channel, within what your provider supports for their country, makes verification easier to complete and builds trust.

To start sending OTPs by SMS, and by voice for Ghanaian numbers, create an Arkesel account and read the developer documentation, or see what Arkesel Phone Number Verification covers in your market.

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