getting a land line phone would probably be much cheaper//
cell phones would be over $20-$30 per month at the least
and a regular phone might be had for less than half that.
there are message services connected to the phone company
but i do not know how much they are. or an answering machine
goes for 20 or so. We have that for our folk society..a
dedicated phone in one of our members' homes...it could
have an unpublished address...or some cell phone companies
have an extra line program that might save some $.

Do you need people to sign up for a tiime slot ahead of time?
some news paper ads have a voice mail phone to call to leave
messages...maybe this is jus tin the personals section...
perhaps you could take out an ad and use the newspapser
phone number for contact! (there are ads where they issue you
your own number to get messages...) how about a pager? I am
not sure how they work or how much they cost...i think they
will take an incoming phone number--but probably not a message.
Aperhaps there is an answering service that could field your
calls for you...not known if they charge a flat rate or per call..

good luck.