You can use media relay for this
Open ports 9300 and 2727 to PBX IP
Open ports 16000-16511 to DSP IP (probs already been done if you have SIP Trunks)
I haven't used the UT Phones for this so unsure, but NT series definitely good. Just put the sites public IP in,
Bit on config required on the PBX, set extensions to remote and set the media relay tab up.
Calum M
ACSS