It is interesting finding. Well, I got my GSM Wifi Sip phone just configured and it works just fine. Including Betamax! I am sure it is to do with the phone adapter.By the way if anyone wants to know which GSM phone! The details can be found here. I must say firmware does need upgrading and one of their rep said it has to be updated by the manufacturer.
http://www.gtek.com.tw/PWG-500.htm