1. She is on the phone, so call back later.

Question

2. She is on the line, so call back later.
3. She is on another phone, so call back later.
4. She is on another line, so call back later.
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Are they all grammatical?

Answer 1

All are grammatically correct, but I'd use only #4. Even better is to ask for the caller's number and say, "I'll ask her to call you back."