USSD support is built into nearly every GSM (Global System for Mobiles) phone sold in the past couple decades. GSM is a type of mobile phone (cellular) network found in essentially every country of the world. In the U.S., AT&T and T-Mobile run GSM phone networks, for example.

Most of Europe uses GSM technology, and many countries in Africa followed the lead from Europe. So on a GSM network, using a phone that runs on the network, you can "dial" a phone number that is a shortened code. After dialing, the phone will connect to a back-end service and get a response. This USSD is a technology that was designed back when phones were text-only and before phones had internet service, but the aim was to provide a way to navigate a menu and enter data (from the keypad) to drive a response from a back-end service.

USSD today overall is not frequently used by those with a smartphone, as the user experience is generally better on a mobile phone app or on a website accessed via a browser. But if a service or feature needs to support those mobile subscribers who use a feature phone, then USSD is one of just a couple ways to communicate with a back-end data service from those feature phones.

As a result USSD is how online banking is offered by a number of banks in various countries in Africa, where feature phones are still used by a good fraction of the population. For many years for many of these banks USSD was the only method in which online banking was offered, though today a mobile app for smartphones is also available from most banks.

Offering USSD requires coordination with the mobile network telecom operator, so essentially that can be a choke point. Machankura is the first bitcoin wallet built on USSD, and it just launched a few weeks ago, so this service might not be permitted in a country that is hostile to bitcoin (and very well could end up getting blocked in a country in which it already operates). We'll see, I guess.

On the back-end, Machankura is just custodial wallet software that responds to the commands from the mobile subscriber's device. They do use a third party USSD gateway service, so they are not a direct customer of any of the mobile network telecom operators directly.

But anyway, the user guide shared in the following post here on SN has screen shots that will show you how Machankura is used:

How To Transfer Bitcoin Via USSD Code #41321 https://thebitcoinmanual.com/articles/transfer-btc-ussd/