MTN on HTC Mteor - Can't call a specific vodacom number: MTN or phone problem?

Hello

I recently gave an old HTC Mteor phone to my girl friend. She's on MTN and put in her sim and everything works except for some reason she can't phone my number (vodacom). It just immediately gives a "call ended 0:00" and it makes the number busy tone. She can phone other people on vodacom, but not me. I phoned my number from a telkom line and it worked fine, so I don't think the problem is with my phone (Nokia E51).

Is this a MTN problem, phone problem or something else?
Are there MTN-specific settings I need to set up on the phone? I did a "reset" on the phone before I gave it to her. AFAIK the Mteor can't receive settings via sms.

there are some problems with ported numbers,donno how it happens but some mtn numbers can't call those numbers...it happend to me when my friend ported his number to voda

Neither of the numbers are ported numbers, so it's not that unfortunately

Test her sim in a different phone?

Works in a diff phone it seems :/