I just activated a Ting SIM today. At first, I tried to give them an IMEI of an iPhone 5s Verizon unlocked. Ting wouldn't accept it. I then gave them an IMEI of an iPhone 8, which was accepted. After my number ported over and the line was working on the iPhone 8, I swapped the SIM into the 5s and the line worked fine.
I bet it's an issue of T-Mobile and their MVNOs only allowing initial activation using a phone that has VoLTE capability. However, the SIM can be swapped into a non-VoLTE phone after the line is activated.