Calls keep cutting when roaming

Hi,

I have been experiencing this problem ever since I ported my existing Telkom Prepaid number to a Telkom Contract number. When ever the network roams on MTN, everytime I make a call, the line just goes mute after 30 seconds or so and you have to make the same cal again. So you can only talk for approximately 30 seconds when roaming on MTN, this is really frustrating.