How does a pc use fiber router, broadband network and sim card to make phone calls and send and receive short messages?

No matter what you do, you should finally send voice and text messages to your phone and mobile phone. Internet settlement will be conducted with the three major operators. It can't be free.

If you are at home, you can install private broadband with telephone service, which is very cheap.

If companies use the top three to buy VoIP traffic, they provide voice gateways.