I was in the voice messaging business for 10 years, so my theatres pretty much got their answering service for free. Having sold the business, I now find myself cheap and stingy when it comes to buying phone lines.
I use the Vonage internet phone service, and set the line to answer immediately. It only allows for 1 minute of announcement, but will answer as many calls as it gets. I keep my messages pretty much to what's playing, what it's rated, where we are, what our admission is and then mention our web site for further information. I can comfortably get a 3-screener into the 1-minute. However, if I had to put showtimes in, that might be a problem.
Messages from the callers can be deleted on the Vonage web site with a couple of clicks, so you don't have to mess with that much.
It's not as nice as my voice mail was, but it's a LOT less expensive, and does the job.