Good morning, unfortunately I no longer have access to the product. I had not tried to configure VoIP at the time, but port-forwarding, although present in the parameters, did not work.
The RJ11 port can be configured and used with a VoIP account for transmitting and receiving calls, or in VoLTE, depending on the compatibility with the telephone operator. Configuration takes place via the router interface at address 192.168.1.1.
If you remove the eSIM from your router, you should still be able to use a normal SIM card. The ZTE MC888 supports the use of a nano-SIM card for network connectivity, as indicated in its quick start guide. Removing the eSIM would simply eliminate the specific network information associated with it, allowing you to insert and use a traditional SIM card instead.
I was able to do a search and unfortunately there is no indication in the data or specification that would suggest a modification or operator. We have various suppliers for this product. Unfortunately, I cannot name any here. If the product does not fit, we also have a return policy. You can find more details on returns or the conditions if the packaging has been opened in the following link: Return policy