PBX Extension over Wireless PTP Link

We have a Nortel Meridian PBX system in our office building. I would like to extend the reach of our central PBX system using a Wireless Point-to-Point(PTP) link and a VoIP gateway to one of our branche offices 7 Km away.

Can I do this Wireless PBX extension from the digital PBX extension port(RJ-11, 2-wire jack) on the wall of my office, to which my digital Meridian telephone is connected, by connecting the port on the wall to a digital VoIP gateway and then connecting the RJ-45 Ethernet connection of the VoIP to the Ethernet port on the Wireless PTP Radio?

Could this be done or I MUST interface my VoIP gateway with the PBX directly to do such wireless extension?

Please help me. Thank you