I would have to say go with a Mitel for an IP solution. Currently they are in the lead. Most of the guys that have IP started with IP and fanagel the "voice" side of it but Mitel started out with voice. I have seen an IP demo where the PBX was in Canada and the guy plugged his phone in to an internet connection and dialed 0. He was connected to his receptionist in Canada. Surprisingly the voice was extremely clear.