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It is because it is free to call any distance, not the quality.

All operators in the US offer unlimited free long distance within the US for several years.

A very large percentage of the country never makes international calls.

The reason for Whatsapp's success is because of the lack of borders in communication. It's not only free in the US, it's free worldwide.

See other comments but literally inside the US calls are not what I am talking about. WhatsApp is used internationally.