it won't matter which carrier you have, since eventually you'll be able to roam on any network.
Nope, sorry. For three reasons:
- 1.) VoLTE on one carrier is not necessarily compatible with VoLTE on another carrier. VoLTE is not plain VoIP - like Skype etc. - where it is a pure "over the top" Layer 7 application that any IP network should support. It is built at a much lower layer in the OSI stack, and each carrier's implementation will be optimized for their own network and may not be compatible with another carrier's.
- 2.) To roam on "any network" (at least in the US) requires your phone to be able to access all the different LTE bands licensed to different carriers. Most phones sold in the US don't because it costs extra money to support the frequency bands of multiple carriers which is pointless when 95% of customers will use the phone for its two-year lifetime on the carrier that they bought it from
- 3.) Also - to roam onto another network, by the way that GSM cellular technology works, your home carrier must have a roaming agreement with the "other" carrier. Generally speaking, the big US carriers have roaming agreements for international use and for remote rural use, but not in domestic areas where they have their own networks. The simple explanation is that if you lose your Verizon signal for a second and your phone tries to go roam onto T-Mobile, that costs VZ a lot of money.... whereas in that area it's more likely that you will get a VZ tower back within a few minutes and not cost them any roaming fees if you didn't attach to a roaming network. TL;DR - somebody will always pay more if you are using a network other than your "home" carrier, and that somebody will end up being you - at a rate that will make it economically unfeasible.
Lastly, if you thought that VoLTE was going to mean that you could just use any given carrier at your convenience, I'm sorry but that's just not how cellular works. In the mobile (GSM and its successor technologies like LTE) world, you have a "home" carrier (who gave you your SIM and sends you your monthly bill) and you will always use your home carrier whenever possible because it's less expensive for them. To use another carrier - even if they have better coverage in a certain area, and your device has the other carrier's frequencies enabled - means that your home carrier will absorb roaming charges and they will pass those along to you. With a markup. So it makes no economic sense for you or your carrier to just let you use the network that has the strongest signal in any given area... or if they do, be prepared to pay out the frickin' wazoo for every time you surf the web on a carrier that isn't your home provider.