Good afternoon,
A similiar question was answered here : TRB500 and VoLTE
And here : RUT-200 does it work with a VOIP phone
And here : VoLTE switch RUTx50
Overall in the device, VoLTE functionality is used to not lose internet connection, once you receive a phone call to the router.
You need to use an external Voip phone, as it operates independently from the router.
The RUTC50’s VoLTE feature is for enabling voice over cellular for the device itself but doesn’t directly support the use case of connecting a wired Ethernet SIP phone (like Yealink) to ring and answer cellular calls through the router alone. Achieving that will require setting up a VoIP gateway or third-party service that bridges cellular calls to SIP endpoints.
Regards,
Arturas