Send short messages by VoIP. The results need to be sent to a mobile phone number before sending. After the transfer. What's going on

There is a server behind these network softwares. When you send the other party's number and short message content to the server, the server will help you send the short message to the other party's mobile phone. This server is connected with the operator's network.

For you, only the traffic (from your software to the server) is taken, and the other contents are completed on the server side. This server usually keeps track of the past, the year or XX messages.