전화하다 1. To press numbers to talk with someone. 2. To talk on the phone. 통화하다 To talk on the phone. When you are pressing numbers if you say "나는 통화 중이다", you looks weird. You should say "나는 전화하는 중이다" or "나는 전화거는 중이다"

전화, noun, a.phone to give a telephone call (to someone) (누구에게) 전화하다. (누구에게) 전화를 하다/걸다 (누구에게) 전화로 연락을 하다. 통화, noun, a talk/conversation on the phone to talk/converse with someone on the phone (누구와) 통화하다 (누구와) 통화를 하다 (누구와) 전화로 대화를 하다.

They're essentially the same words, but if you want to pick apart the small differences, 전화하다 - A phone call/to talk on the phone 통화하다 - To make a phone call

Nothing different. Just another word.