Perhaps a Grandstream GXW4104 as already suggested, available for under $200 on amazon. Similar devices from Patton, Audiocodes,Cisco and Sangoma, they all work much the same way providing a SIP trunk at the FreePBX to the PSTN connections at the Nortel end, if the handsets are analog (plain old telephones) add 8 ports of FXS as SIP extensions on the FreePBX
Generically you are looking for a 4-port FXO (means a trunk not a phone) SIP ATA (means analog telephone adapter) gateway, specifically you will need to get one that is as simple as necessary for you to setup.