There are lies, damned lies, and SLAs. Personally I only find an SLA useful if it is worthwhile. Most of the SLAs out there aren't. And for good reason. You should probably offer one, but like a smart company shouldn't make the burden too bad.

Suppose someone doesn't respond to a page. Is it because they were too far asleep to hear the paging device? Because the paging device didn't work? Because some other problem kept them from working on the page remotely? Because their carrier blocked the page? Because you broke down? Because the problems in their system kept them from sending you the information in the first place?

There are a lot of points of failure. And your service is not one of the more likely ones to break. Furthermore if there is a dispute, whose records win? They didn't respond to a page, your records say they never sent the page. They blame you, how do you resolve that?

Therefore I'd suggest offering an SLA, but make it be something like, "If you missed a page and are convinced that it was our fault, we'll refund the last X months." From your point of view it is a no questions asked refund policy, that carries with it the consequence that that person is not allowed to sign up for your service. (Unless, of course, you're convinced it was your fault they didn't receive their page.) But whatever you do, be careful not to accept potential liability for something that likely was their problem.

I would also suggest that you share best practices. For instance an important one is that companies need to provide a well-defined escalation path. Recognize that humans fail (whether because of not waking up, being in the process of driving, etc) and so people are unreliable components that need a fall-back mechanism. The act of educating your clients about things like this will help them avoid problems that could cause them in an imperfect world (ie the one we live in) to become unhappy with you.

At a former employer, it took weeks to negotiate the terms of an SLA with a solution provider and it ultimately fell through. We glossed over several vendors because of the lack of one (it's usually negotiated, not pre-written like a privacy policy or T&Cs.)

However, what ultimately made the deal was not an SLA, but a new vendor that showed substantially deeper pockets than we expected. A meeting was organized with their sales guy and FOUR suits showed up in a black sedan. Most gangsta display of power, and our glass cubicles were gassed down with the musk of cigar, Brut and Drakkar Noirs.