My calls keep dropping due to a pop up message, how do I stop this?

Lately (as in everyday for the passed two weeks), my calls have been dropping and the message that pops up is "call failure". I always thought that it was bad reception as my boyfriend has quite poor reception in his area. I recently noticed though that the calls are only failing once I've hit the 2 hour mark of talking. I am in a long distance relationship so I'm calling from NZ to Aus if that has anything to do with it. It only started happening once I switched carriers when I got back to NZ 2 weeks ago. Is this happening to anyone else? How can it be fixed?

Are you using the cellular network for the call? Or FaceTime?? Some other VoIP app like Skype? If using cellular... it's a network limitation imposed by your provider... you can try alternatives - FaceTime, Skype, etc...