A couple of somewhat crazy suggestions, take em or leave em.
1 - Is this a phone with a cord between handset and the hook per say - with an RJ114P4Cconnector? If so, two of these will be mic and the other two the speaker. I can look up which is which if need be. If you don't need the phone to work, then just connect the handset to a socket - it will fit into an RJ11, an RJ12 or an RJ45. and then just wire the mic up from there. You should be able to split the signal if the phone needs to still work.
2 - If the above won't work, then assuming that the phone is not connected to the public network at the time, connect the phone to another phone - wire up the speaker in this second phone as per the above, just use the speaker connections instead and run it into a line input. Then connect the two phones to each other using one of the "telephone intercom" circuits floating around on the net, it can be as simple as a power supply, a limiting resistor and perhaps another component or two (can't remember off the top of my head).