It's available - but using Voice over IP (VoIP) and the data transmission facility of your mobile rather than traditional voice calls.

VoLTE is a method of voice calling over a mobile data connection. If you google something like 'volte single number multiple devices' you'll find several operators offering a single number on several devices - or planning too.

This explains how it works:
Ericsson: Multi-SIM: One Number, Multiple Devices

It could also be done using an app, in the same way that message sent to an apple or google account is received on all devices. You'd just respond to an alert and start a VoIP call. I don't know if anyone offers such an app - I expect so.