pianotuna
NO ONE told the OP to buy a 3G phone.
They DO make flip phones which have 4G WITH VoLTE capability. Models are pretty limited but there ARE 4G WITH VoLTE flip phones available.
To MY point, absolutely NO cellphone PROVIDER is going to intentionally sell you a non 4G VoLTE phone at this time PERIOD.
If one purchases through their cellphone provider they WILL get a cellphone with at min of 4G VoLTE capability INCLUDING A FLIP PHONE as the OP was inquiring about.
Now, if one were to go "off the grid" and go rogue and buy a unlocked phone, yes, the possibility does exist that one may end up with a 3G/4G (non VoLTE) doorstop.
And by the way, VoLTE was not available in 3G, it was brought out in LATER 4G systems. If one was an early 4G adopter, there is a good chance that your 4G phone may not have VoLTE capability and that phone will also have to be replaced when the 3G networks go dark as the new minimum supported standard is 4G WITH VoLTE.
One also can search the model numbers of the phone you are looking at and see if the manufacturer offered it in 4G VoLTE. Most of the cell phone companies also have a supported phones list to help determine if the phone you are looking at meets the current 4G WITH VoLTE requirements.
My recommendation to the OP still stands, by the flip phone that your Cell phone service offers, it WILL meet the minimum 4g WITH VoLTE requirements and IF the phone does not work, the Cellphone company you bought it from MUST make things right.. Buying from a third party while you can save money or buy a better phone can and does open you up to the cellphone provider to stick it to you if the phone doesn't work on their network (I learned that the hard way).