Ubuntu Touch Planning Path For VoLTE/4G Support
Currently Ubuntu Touch doesn't support VoLTE/4G but considering 2G and 3G networks around the world are beginning to be disbanded, they need to begin towards supporting at least 4G.
In the United States and elsewhere network operators are beginning to dismantle their 2G/3G networks and thus Ubuntu Touch needs to quickly catch up in handling VoLTE/4G to remain relevant. The UBports Foundation has contracted Sysmocom to begin working on the software support so VoLTE ultimately can work under Ubuntu Touch when running on phones with capable hardware and firmware.
From Sysmocom's research so far they are planning on adapting the Doubango IMS framework for their VoLTE support given its open-source status and being the most complete and re-usable codebase they have found so far.
UBports so far has enough funding for the VoLTE prototyping to happen and thus they are moving ahead there with an initial focus on getting VoLTE working on Ubuntu Touch for the Volla Phone. After that the second phase is coming up with the proper/complete implementation.
More details on the UBports plan for supporting VoLTE with Ubuntu Touch can be found at UBports.com.