They keep using that word "credit". I don't think it means what they think it means!

To begin with, m-Pesa emerged fifteen years ago after Safaricom, the leading mobile telecom provider in Kenya, noticed that mobile subscribers were transferring their airtime credit (i.e., credit for voice minutes and texting) as a way of transferring value person-to-person. But m-Pesa service is (and always was) for fiat currency (i.e., Kenyan shillings). There's no "airtime credit" transferred using m-Pesa. It's for transferring from your KSh. Actually, thinking about that -- there is, like a short-term loan service, where using the m-Pesa service you can borrow some funds, and when those KSh are added to your m-Pesa balance those fiat funds can be transferred. But you aren't paying with credit (like you might with, say, a "credit card").

Now there are other mobile money networks (over a hundred and fifty, or so, significant ones), ... but they all work in this manner, where it is fiat from subscriber/account-holder balances that are transferred.

Though there is (at least) one exception -- Grass Roots Economics' Sarafu mobile money, which is a Community Inclusion Currency (CIC). But CICs are more social experiments where concepts like basic income, demurrage, and closed loop / circular economy approaches are tested, and they are not mobile money networks, per-se. At least not at their current levels of traction.