Motorola V600

I have been wondering whether and unbranded, unlocked v600 would allow a data call? I noticed that if you call a fax number like a voice call the fax machine would answer. Now if you set your phone to Voice first then fax you hear a message from AT&T stating they do not allow calls of this type. The phone is some how send a signal along with the call to notify AT&T that this call is a data call. Does any one have a unbranded unlocked phone on AT&T to try and make a voice then fax call?