Informal:
* On se parle plus tard. (This is the most common and natural way to say it.)
* À plus tard! (This is a very casual and friendly way to say it.)
* On se retrouve plus tard. (This is a bit more specific and means "see you later.")
Formal:
* À bientôt! (This means "see you soon" and is more formal than "à plus tard.")
* On se reparlera plus tard. (This is a more formal way to say "we'll talk later.")
You can also add a specific time or activity to make it more precise:
* On se parle après le déjeuner. (We'll talk after lunch.)
* On se voit plus tard ce soir. (See you later this evening.)
The best way to choose the right phrase is to consider the context and your relationship with the person you are speaking to.