DIR-615 has SIP
http://www.support.dlink.com/emulators/dir615_revC/310NA/adv_dmz.htm
So does the DIR-625.
http://support.dlink.com/emulators/dir625/109/Advanced_Firewall.html
So its not a compatibility issue. What ports does your VOIP use? Is it hardware or software based? If the former, what hardware, if the later, what software?