I have little complicated inquiry, at least to me

I have Nortel BCM 50 which is working so far fantastic. Only concern is about telephone lines cost. I have 1 main line and then 11 roll over phone lines which costing me around $500 monthly.

I have install this PBX but now question is, once I buy new DID and add more channels, if I configure it in FreePBX, I need SIP software to connect to my server.

Can I configure my existing IP phones (nortel I2004) with this server for incoming and outgoing lines as well SIP software, both at the same time?


Is there anyway the I can transform the DID number to PSTN or dialtone (with added channels)via pap2 device and then insert telephone line directly into BCM 50?

If yes, please help how ?
Will my main line work just like regular phone roll over lines ?
Do I need more then one PAP2 device to configure all sub channels I added on DID ?