Is the existing cable between the buildings a direct run? Is it only a single pair or multipair? (Take off the wallplate with the phone jack and look inside). If one or more pairs in a cable bewteen the two buildings is unused, a simple wired intercom can be used, separate from the phone.
Even old telco wiring was done with a quad cable (red, green, yellow, black conductors). The red/green is all that is neded for a single phone line, so the unused yellow/black pair can connect two IC stations. An example of a simple two wire (one pair) intercom is the Intertalk WC121, about $30 from Amazon and other websites.
More recent telco wiring uses "category" wire with 4 pairs...more options for intercom stations, and/or aditional phone lines as needed.
If there's only a single pair, or the line is "multed" (two lines tied at the Telco post or crossconnect block, and not a direct run) you have fewer options. Would a wireless intercom work at that range?