Changing Phones on Straight Talk

I purchased the Nokia E71 but I can not get service at my workplace. Straight Talk informed me that the E71 piggybacks on AT&T. I left AT&T to get away from the poor service. My coworkers have other model Straight Talk phones and get excellent service. Can you please tell me what phones Straight Talk offers, that preferably uses Verizon or T Mobile? I would like a phone with similiar features of the E71, especially Navigation. Also, if I trade in the phone, what is the procedure to activate the newer phone and maintain my cell number since i also recently transfered it from AT&T to Straight Talk?