The lack of VoLTE early on was not quite as simple as missing software as there was, at the time, not really any agreed upon VoLTE architecture or implementatio

The lack of VoLTE early on was not quite as simple as missing software as there was, at the time, not really any agreed upon VoLTE architecture or implementation. IMS has existed since before LTE, but carriers have been very reticent to implement it because the choice to architect around existing internet protocols actually leads to a relatively high degree of complexity from the perspective of legacy telcos. For most of the early years of LTE, most carriers just didn't support VoLTE as an intentional business decision, to avoid the expense of deploying infrastructure. Those that did mostly deployed "lite" VoLTE stacks that were simpler and less capable than the IMS architecture, and this is the big issue that leads to the situation we still have today in some cases were given phone models are only capable of VoLTE with specific carriers, due to those carriers using a non-standard architecture. Most LTE phones could do HSDPA+ during voice calls but some couldn't. I'm not sure why, it may have just been a simplifying decision by the vendors. This was more of an issue in the very early days of LTE, e.g. my hazy memory is that the HTC Thunderbolt completely dropped data during voice calls.