Crashes alot with Verizon works on Att

I am a 3CX Reseller and Im running an Iphone 4s with Verizon and one with ATT. Side by side they work in office on wifi but Verizon crashes here and there, ATT phone barely crashes. When I take them off the wifi and use the out of office connection the ATT iphone works flawless but the Verizon cant connect and frequently crashes trying to connect.
FIX THIS!!!!

Blue Jays Band about 3CXPhone for 3CX Phone System 12