In the past I have worked at 2 different companies that provided employees with cell phones, both operated pretty much the same. 1 job I was a delivery driver, they provided us with a cell phone and basic plan (before texting) which the company paid for. All other company phones were on the same plan so it didn't take any minutes to call any other phone in the company, your monthly plan included 400 minutes which you could use for personal calls. If you went over your minutes the overages were directly deducted from your paycheck.
Second job was a construction company where only the foremen were given company cell phones however other than that it operated exactly the same. Set number of minutes provided by the company, you go over you pay for it. This way seems more effective for a landscaping company, I don't really see any reason that every person on a mowing crew needs a company phone.
Another option is to put a company paid phone in each truck and whoever is running the truck each day is responsible for the phone for the day, that's how we do it at my current full time job on the ambulance.