Moz 4 Newbie Poster

This story can be quite long, but the short question is - has anybody made an outgoing vpn connection with a Billion 7404vnpx or 7401vgpr3 and managed to maintain the connection for more than 20 seconds?
I have been banging my head against a wall trying to get my 7404 to hold a vpn connection, but it won't. I did it successfully and easily with my 3G USB modem, and later just as quickly and easily with borrowed Billion 7301vgp, Billion 5102s, and Netcomm NB6Plus4W.
Because those other modems worked, I (stupidly) figured the 7404 must be faulty, and bought a 7401vgpr3 to get me through until I could get the considerably more expensive 7404 fixed. But that modem also dropped the vpn after 20 seconds.
Billion haven't been much help.
It seems they may have problems with their 74xx chipset modems. Can anybody add any more info to this? Or perhaps tell me why the connection drops out after exactly 20 seconds EVERY TIME??
Thanks