The system was never designed to handle a call type like that. Actually, that type of call would be mutually exclusive, because you can only have one call in process at a time on an analogue line. Now you could get voice mail from the telco, or add a 2nd direct line, but that would probably defeat your purpose, and would cost more.
Now if the client system has other capabilities, like DID or PRI, then you might be able to rearrange some things to provide that feature. What does the system have for telco services? (POTS, DS1, PRI, DID on analogue trunking)
....JIM....