[andy]

Well, most of the global SIM providers seem to have brought several elements together themselves rather than purchased the whole thing off the shelf - SIM card with roaming agreements, switches, carriers to connect the calls, back office and billing systems ...

So I think you have a choice between doing this development work yourselves, or becoming a reseller for an existing platform; despite your comment it sounds more like the latter to me. Or perhaps even look at a whole set-up as a prospective buyer or new investor. Maybe it depends how many customers you expect to have.