Before buying the additional module, I had considered doing this using the built-in recording slots... the only catch is that they require that they be recorded via the telephone. This actually isn't that bad though - I have this little device that costs a few bucks from radio shack that plugs inline between a handset and the phone (worst case a cheapie walmart phone would suffice). There's a switch on it - one position lets you record to the mic in on a tape recorder or PC - but the other position lets you play sounds from a PC or recorder back into the phone. I've used these to play messages back to people over the phone - I figured they'd do a pretty decent job for the exact purpose you're talking about.
Be careful though - it seems for some people when doing these custom recordings, somehow the built-in recording slots were overwritten by mistake (elk problem) - it seems to require being sent back to elk to fix.
I personally just added the elk129 - and didn't do anything special with the wiring for the capacitors, etc... mine just works.