What does the SMS service remind the unconnected caller mean?

It means there is no network connected to the calling number.

There are several reasons for "not connected": 1. The other party's mobile phone signal is weak or has no signal. 2. The other party just picked up the phone and started talking. During this period, it always prompts that the connection is temporarily unavailable. 3. The other party is in the process of sending text messages. 4. The other party has set call restrictions. 5. The other party sets your phone number in the incoming call blacklist (caller blacklist). 6. The other party rejected your call.