Supposing

Use a telephone pickup coil. Lots of places used to do them over here. I'm sure Rat Shack or whoever probably do them over there. It's just a coil with a lot of turns on it, and a little rubber sucker. You just experiment and find the best place on the phone body or handset for it. Probably not as good these days as when phones had dirty great hybrid transformers and such in them, but I think there is still enough radiation and enough wound components in there for it to at least work. Some answer machines - some of the better Panasonic ones from a few years ago, for instance - also had a conversation recording facility.

Arfa

RadioShack also has an adapter that that goes between the handset and the telephone. A second cable plugs into the recorder's mic input. It's more-expensive than a coil, but a lot more convenient. The only reauirement is that the handset have a modular cord (one with connectors that unplug).