1. When somebody telephones you from abroad, you should answer the phone at once.

Question

1. When somebody telephones you from abroad, you should answer the phone at once.

2. When you are telephoned from abroad, you should answer the phone at once.

3. Being telephoned from abroad, you should answer the phone at once.

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Are they all grammatical?

Answer 1

1. When somebody telephones you from abroad, you should answer the phone at once.

1 and 2 are fine; 3 is very awkward and not used.