When you say "phone line," do mean a dial-up circuit, or a dedicated, leased circuit?
If you are doing dial-up, then you are going to need a telephone hybrid with a good null. The null provides isolation between the send audio and the receive audio to prevent feedback and hollow sound. The standard phone hybrids in the broadcast world are the Telos Hx-1 and the Comrex DH-20. Be sure to be seated before pricing them.
Hybrids are expensive because to get a decent and stable null requires DSP to get the job done. Analog hybrids are useless junk because the null is poor and varies from call to call.
Does it have to be a phone line for your purpose, or are you simply trying to give intercom access to someone at a remote location?
If it's the later, you could look at something like Clear-Com's LQ interface which lets you bridge separate intercom systems over an internet connection.