No! Approaching a girl while she is talking on the phone could be the worst idea ever. She might consider you to be an outsider or as a threat to her. Well, approaching anyone while he/she is on the phone does not sound very civilized anyway.
Hence, instead of ruining your first impression this way, stay patient and wait for her to end the call. If she does not end the call and if you are running out of time, consider it to be your unlucky day and delay your plan of approaching her. Better wait, than to ruin the first impression.