I wouldn't keep the phone either..
I'm with Mr. "not this old chestnut...". Now my job was even cheaper; at one point, the boss just started giving everyone that rang in my cell # and telling them to call it instead. It hadn't occured to him this would be a problem....(he's an alright boss, but pulls some real boners once in a while..) After having the phone ring off the hook for like half an hour, shutting it off, and finding I'd missed like 20 calls in the next hour, I finally figured out what was going on and told him to cut that shit out right away. He was like "I'll reimburse you" but I pointed out
a) overage came to $42/hour (I'm in the US so I had 450 minutes included, but overage was $0.70/minute or so)
Also b) (More importantly to me) I'm an hourly employee, and since these guys don't work in my building, they would not know if I'm at work or not, and would inevitably phone me at home. Even though standard hours are 9 to 5 and sometimes the place I work runs 10 to 6 instead, I've been at work as early as 7AM (only once or twice) and as late as 8PM and the phone would ring on and off the entire time. Generally I haven't answered after 6 o'clock since the place is supposed to be closed; my favorites were when people would call, hang up, call, hang up, like 20 times in a row. Invariably if someone gives in and picks up after 10 or 15 times, the person is calling to ask if the place is open (!). You'd think they could guess "no" from noone answering the phone the first dozen times. (The boss and guys I work with can call my phone, but they know they don't like being called at home either, and only call if they really need to.)
Anyway, indeed if I were given a company phone, it's unwieldy to carry two phones, and I'd probably leave it at work if I wasn't actually allowed to use it. Personal calls are a small price to pay for having someone on call, typically on call without compensation.