This is only really recommended for those times when you just need to get on for a few minutes to check email, etc. Most campground offices don't have much understanding of computers in general, so will assume you mean 5 minutes or less when you ask to use their phone line. They don't realize that it often takes that long just to set up your computer for the local phone number and get through to it. You're generally left feeling guilty for tying up their business line or, worse yet, asked to please terminate your call so they can get back to work. For this reason, I don't recommend this approach unless you're really out of options and feel a strong need to check your email regardless of the consequences.
One thing that many of the campground offices don't realize is that they've got a separate phone line for their credit card machine. If this is the case, you can simply plug into it's line and you won't be disturbing their business unless they need to use the machine.
As I pointed out in the section on modem-friendly campgrounds, this is what modem-friendly means to some of the campground directory authors. One of the directories that differentiates between "phone line available in the office" and "phone lines you can use for a reasonable length of time" is the RV LifeStyle directory -- for $21.95 per year. Click the little Cruising America logo to go check it out. A FREE index is available from RVers Online -- click below.