DIDs versus SIP Trunking for conferencing?

I am looking to setup a simple low cost phone conferencing control system.

I like the features in FreePBX and would like to try and use it.

My question is, Can I use the 4 individual DID numbers I currently use on another platform with unlimited inbound channels with FreePBX to do this?
Or do I need to add SIP Trunking and multiple lines?

I do not need to use any other functions like outbound calling.

Thanks for your support.

How do you currently get your four DIDs with unlimited channels, if not via SIP Trunking?

Hi Martin,

I apologize in how I phrased my question. I have one DID set up on a Freeswitch setup.
I purchased 3 more today to test a couple of other setups with the hopes i could use all 4 with one type of PBX or another independently.

I am asking my question here in the hopes my assumptions of now not being able to was wrong.

Not needing outgoing lines I was trying to figure out the cost of just running a conference system for less than $250.00 a month in SIP Trunk lines for incoming calls only.

My Freeswitch works with one DID and 20 plus calls in multiple rooms now. I was trying to find a PBX or other system that I could just add more DIDs to add the function of more rooms and callers for a reasonable monthly cost.