Changed SIM - now can't make calls has an iPhone

My wife bought an iPhone in Indonesia, where the provider is Telkomsel. On a recent trip to the UK she bought a local SIM card so as not to run up huge roaming bills. The first one didn't work but the second one did (Orange I think). On the way back we stopped over in Singapore and replaced the Telkomsel SIM. Strangely she could not make outgoing calls through Singtel (I could), although all other functions worked fine (receiving calls, sending and receiving SMS etc...). After experimenting we found she could make outgoing calls through StarHub. Now we are back in Indonesia and she can't make calls through Telkomsel any more.

I tried swapping the SIM's from my iPhone, and it appears that the problem is with the phone, not the SIM (her SIM works in my phone, but my SIM does not work in her phone).

It seems like changing the SIM in the UK has changed some internal setting on her iPhone. I've tried resetting her network setting to factory default, but this has not changed anything. Anybody got any suggestions before I take the phone back to Telkomsel?