Natural stop
Other than trying out parental control apps, if you consider yourself brave enough, you can try doing it by yourself. If you come in terms with your kid to a common set of hours that you both agree on, you are to be considered a very lucky parent!
You can try out discussing a certain amount of time for playtime on the telephone, and if he wants more, you can cut the minutes from the next session. Like this, not only the child will get a sense of value over his time with the telephone, but he/she will also learn how to respect your word.
