We have the same problem with MTN, clients use MTN 3G cards to VPN into Verizon, about 9 out of 10 times it does not work, it works 10 out of 10 times if using a Vodacom 3G connection. The funny part about MTN and Verizon is that they are the same now after MTN bought Verizon... We have had this problem for months now, had MTN and Verizon involved and they just cannot find the problem, DONT USE MTN.