Is this correct grammar to say when is the best time i can call you?

The grammar is technically correct, but it sounds a bit informal. Here are some better options:

More formal:

* "What is the best time to call you?"

* "Could you please tell me the best time to reach you by phone?"

More casual:

* "When would be a good time to call you?"

* "What's the best time to catch you?"

The best choice will depend on the context of the conversation and your relationship with the person you're speaking to.