My situation is not quite the same as yours - I'm on the road 5 days a week, as are the majority of the rest of my company's consultants. I used to get a flat $50 a month to cover my cell phone, but when the company asked me to add email access to my personal phone I told them that they'd need to cover the whole thing....so now they pay $120 for full BlackBerry access for me. It was either add it to my personal phone or carry it plus a company BlackBerry....and my business cards have my personal cell number on them anyways.
Are the phones your staff use company phones or personal phones? The company at which I'm deployed now has a number of people who travel sporadically, and they have a loaner phone pool. In theory, they're only to be used for outgoing calls, so that the current holder of the phone doesn't end up with incoming calls along the lines of "but I called this number last week for Joe/Sally/whomever," but I don't know how well that works.
The loaner phone idea might work better than the prepaid idea. I like the prepaid idea in concept, but what happens when someone heads out of the office and then realizes that they only have 4 minutes left on the "company" phone? They'll use their personal phone, then grumble when they get back about "Dingo gave me a phone but it only had 4 minutes on it, but he's paying for all of the sales guy's phone. What's up with that?"
That said, if there are only two people who travel periodically, you might be able to make an arrangement where you pay half their monthly bill or something like that. Even if they've gone to the latest unlimited calling or whatever kind of plan, you're probably only looking at $50/month apiece. Just remember that you'll only pay for the primary line, in case they have a family plan/text messaging/ringtones or other crap that they try to use to pad the bill. At a small company, that's probably less likely to happen, but you never know.
Let me know what you decide....always interested to hear about policies like this as they're something I must follow.