while I was doing
It can also be used to express that the action in the first clause continues while the action in the second clause occurs.
'가' is sometimes omitted with no change to the meaning.
This expression is used to indicate that the action in the preceding clause continues while the action in the following clause occurs.
The subject of the preceding and following clauses must be the same.
The sentence is ungrammatical because the subject of 전화가 왔습니다 is 전화. So it has a different subject from the preceding clause.