Informal:
* "Ok, falamos depois!" (This is the most common and straightforward way to say it)
* "Até mais!" (This literally means "until later" and is a very common way to say goodbye informally)
* "Falo contigo mais tarde!" (This literally means "I'll talk to you later")
Formal:
* "Muito bem, até mais tarde." (This is a more formal way to say it)
* "Ok, até breve." (This literally means "Ok, until soon" and is a slightly more formal way to say goodbye)
Other options:
* "A gente se fala mais tarde." (This is a slightly more casual way to say "we'll talk later")
* "Te ligo depois." (This means "I'll call you later")
The best way to say it will depend on your relationship with the person you are speaking to and the situation.