Carol - Do you have a retainer for this client or is it project priced?
I would be inclined to say that any time your client has his number forwarded to you is billed at your regular hourly rate (assuming you have a retainer and/or some type of support contract and you're not project pricing). I say this because a phone ringing every 5 or 10 minutes would shoot my concentration all to heck and I wouldn't be able to get anything else done during that time frame.
If you think you would be able to get other work done during that time, you could just charge time for each phone call according to your normal billing methods.
If you're project pricing, this is very difficult to do, because you don't know the frequency of calls. If you know the approximate duration of calls (if they're very similar in scope for customer-service type questions, or if you're mainly going to be acting as an answering service and taking messages and determining if they need to be relayed to the client immediately or if they can be dealt with later), you could figure out a "per-call" rate of $X, include x number of calls per day/week/month in your base price, then say something like "calls over x per day/week/month" billed at $x per call" or something like that.
Good luck with this one - I don't offer answering service because I don't like to be chained to my desk and also because figuring out how to do it would give me a headache.