I'm looking for some technology solutions that can help us manage our volunteer rotas. 

We have weekly bike rides and have a pool of volunteer pilots who we email every five weeks to ask them to phone in to our rides coordinator (blind, 80 and doesn't use technology) with their availability.

As the pairings of visually impaired and sighted people change each week, we have to set the tandems up specifically each week. So...we're looking at how technology can help us to get the rotas - I'm aware of 3 rings and I know there are broadcast SMS services - but I don't know any more. It's only for 30 or so volunteers. Any advice of small, cheap systems?