How to open a call waiting for a telecom mobile phone?

Hello, telecom users can edit SMS 650 or "KTHJDD" and send it to 1000 1 to start call waiting.

Call waiting means that when you are on the phone and a third party calls, you can hear a "beep" call waiting tone. At this time, you can choose to answer the third party's call before hanging up, refuse to answer the third party's call, or answer the new call after answering the current call (the other party did not hang up).