I think what you are trying to do is a connect a SIP ATA adapter back to the main ESI 200? Correct?

IF so you could use an existing analog extension off of the 200 and use either Multi-tech or Patton adapters. As for a SIP ATA that works with the main ESI 200 would need to talk to ESI regarding that, I know they had a generic SIP extension license but we could never get it fully correct and abandeed it. Maybe someone else can chime in.

You would also potential crack and tap the speaker on an IP phone and send that to an amp to the speakers.