KGI Securities analyst Ming-Chi Kuo seems to suggest that the upcoming Apple Watch Series 3 might support VoIP instead of traditional phone calls, possibly to avoid user experience issues and also due to the lack of internal space within the device for a proper SIM slot.
https://www.notebookcheck.net/The-Apple-Watch-Series-3-could-do-VoIP-instead-of-direct-phone-calls.241852.0.html