What are the common reasons for not connecting?

First, the other party's mobile phone signal is weak or no signal.

Second, the other party just made a phone call and started talking. This period of time always prompts that it is temporarily unavailable.

Third, the other party is texting.

Fourth, the other side set up call barring.

Fifth, the other party will set your phone number to the blacklist of incoming calls.

Sixth, he refused your call. ...