error 628 is a classic modem error - not specific to 3G
It means the remote user disconnected before the connection could be established
In real terms it can mean several things
remote system not answering
bad line slow data speed
and heres possibly the key one....when you get it with a 3G connection its often because for some reason the dial up software has switched to attempting to use the wrong modem (e.g. an internal laptop modem)
Check the settings for the connection and make sure the 3G modem is the one selected. If it isn't, then correct it. If it is, then you've probably got a dead modem