if you have the 24 extension in already then it is full, you will have to look at a new system.

astericks is a free sip pbx, but you will need a server, sip phones and line interfaces.

A older kxtd 1232 could do what you want. other than that you will need a tde or NCP


“I have not failed. I've just found 10,000 ways that won't work.”