The problem is solved... It was after all a problem with ports being blocked like annacat suggested.
Both IPSUBs had public IPs but the phones were getting private IPs from DHCP... I ended leaving one IPSUB outside the network and the other, inside... the phones in the LAN will connect to the inside IPSUB, and the phones out in the field will connect to the the IPSUB with the public IP. This way I don't have to deal with ports at all.

Thank you all for the brainstorming session!

~Francisco