Lets talk a bit of theory:
Mpesa is owned by Vodaphone UK and managed locally by Safaricom. For this argument, lets treat Safaricom and Mpesa as separate entities.
Suppose Vodacom TZ uses exactly the same Mpesa service that will allow sending money from Mpesa KE (Safaricom) into Mpesa TZ (Vodacom), that will make interesting cross border trade between Voda... related subscribers.
Lets take it further (theoretically). Suppose Orange KE managed to convince Vodaphone UK to use the same Mpesa. So Orange KE and Safaricom users could easily send money into each others mpesa accounts. If this would become hugely successful to the point where Zain ditch Zap, Yu ditch Yu Cash and they all use Mpesa.
Then in that case, it wouldn't matter which network you are on coz you could still use Mpesa.
If that is very far fetched, then we could settle for a 'pesapoint' of mobile money.