i use straight talk and they keep telling me I will need to update to a volte phone soon. But when browsing phones I'm not seeing a volte search criteria or filter so I'm confused what I'm looking for and what is available to me. I really would want a compact andoroid phone but don't know which are compatible with straighttalk and are volte enabled.
It is likely because they use Verizon's network and they are not allowing the use of other phones soon here. This would lead your straight talk service to be changing due to it running on that network. They just don't tell you that outright. Why, who knows.
Alas, there is nothing you can do about it. You will need a phone that will work for there network if you wish to continue to be on their network.
Here are some articles on all this as well as a list of devices that work on VoLTE.