Ask HN: How do you replicate/initiate a USSD session in a web app?

I am trying to replicate a mobile money top-up session in which a user dials a USSD number and then chooses an option from a USSD menu and enters an amount, followed by a PIN. I'd like to replicate the top up process on a web app such that a user could say, enter their number and amount and then enter their PIN which would also result in the same top up as a USSD session. I know USSD is somewhat, if not native to a handset and also that I'd need some info from the network/service provider. I want to know what I'd have to ask them for.