Formal:
* Give me a ring.
* Please contact me by phone.
Informal:
* Give me a bell.
* Give us a shout.
* Give me a buzz.
* Drop me a line. (This can also mean "send me a message")
More specific:
* Call me back.
* Call me later.
You can also use the phrase "get in touch" which is quite versatile and can be used in both formal and informal situations.
The best option for you will depend on the context and your relationship with the person you are talking to.