Check the rates for local calls before you start putting coins in, as the rate and permitted length of call varies from one pay phone provider to the next. If using a credit card or calling card or calling collect, ask the operator for information about rates before dialing.
Rates for the use of phones can vary widely, and while providers must post a rate for local calls with coins, they do not have to do so for calls made using a card. These fees can be quite expensive. If you know you're going to have to use a pay phone ahead of time, contact your long-distance provider to find out the best way to contact them directly from a pay phone. Keep the access codes or toll-free numbers handy for the provider you want to use.
In terms of physical safety, only use a public payphone that it is an area with plenty of light at night. If you are making a call in an area of town that looks unsafe, take enough coins with you to complete the call, but lock your wallet in your car. Public payphones have become less commonplace because of the popularity of smartphones, so the remaining ones may be in vulnerable spots.