The only actual VoIP phone with a SIM card I'm aware of is the Yealink T74LTE which for a long time was exclusive to Verizon's business phone service but has recently become available for general purchase, those can support both SIM over LTE data and VoLTE (which is also basically SIP but over a prioritized channel).
It looked like an office desk phone but instead of Ethernet, a SIM card goes only. Some came with a cordless handset with it too. seeing
Eww, I hate that such a thing exists. Everyone involved in its development needs their decision-making privileges revoked for anything more consequential than breakfast.
There is no good reason for an internet connected device to be cellular-only unless it's intended to effectively never be in "home territory". Every other such device should at least have WiFi so it's useful in areas where there is no cell service and doesn't need to use limited cellular spectrum even wheb there is service.
There is also no good reason for an internet connected device that's not expected to move while in operation to not have an ethernet port. It's unfortunately common amongst consumer-targeted devices (e.g. Nintendo Wii and a surprisingly large number of printers) but it's bad nonetheless. If it can reasonably be wired it should be wired, period. If it doesn't move and isn't in a weird spot, you need to have a really good reason why it can't be wired to justify using WiFi and "I don't like wires" has never been and will never be such a reason.
The reason why this product exists is for homes in places where there is no telephone or cable tv wiring to the house.
This is a massive part of the world and part of why the countries that didn’t have a wired setup in many cars jumped straight to wireless.