How about to use 2 Asterisk system interconnected through Wireless solution point to point
one system should be for ISDN BRI gateway with Digium PCI card and the other server for extension voice mal and so on.
for covering distances it will better to use Motorola Canopy web site or Airmax from Ubiquiti (web site http://www.ubnt.com/airmax).
It should be as the following
PSTN/ISDN BRI Lines < -BRI-> Asterisk< -LAN-> / Wireless point to point / < ->LAN Asterisk customer side registering handsets, IVRs, voicemail, etc
In general these system operates on 900 MHz 2.4 GHz, 5.2 GHz, 5.4 GHz, and 5.7 GHz and can cover a long distances depending to environment.
Please be advise to check local regulation about using wireless system (frequency operation range, authorization and others.
Mc GRATH Ricardo