I'd recommend getting her to contact her provider, when calls are made it's the caller that initiates a path for the call to take to get to the destination.
That being said, the fact that it's the same from her mobile would pinpoint something to do with the handset potentially. Does your handset have call blocking features? Would you be able to factory reset it so that the settings are default and try this again?
I think it must be from her end too. I do have call blocking but it does not block the call for she can leave a message when my answerphone kicks in. I think her call just hasn't enough oomph to trigger my ringer. I'll think I'll get her to contact her provider.