How can I inject audio into a phone line

BAsically you should use a transformer to feed the line, in series with a cap of 1-2 uF. Since you are using a 'phone as receiver, a small driver trafo from any old transistor radio will probably do, any decent line transformer will be excellent.. Feed from the walkman with a series cap.

Your 'phone will need to be powered though, usually from the line.
Try a 9-12 V from a battery pack connected to the 'phone side of the trafo /cap combo. Try 1kohm in series with the battery in order not to quench the audio....
Is something like this on the right order?