VoLTE not working on Oneplus 5T


    I recently switched from an Iphone 8 (bought through T-mobile) to a Oneplus 5T. The phone is great however I don't seem to be getting VoLTE. I am able to connect to LTE networks and have the setting "use VoLTE when availble" set to "ON" however whenever I make a call, the network seems to drop down to H+. Any suggestions on how to resolve this? Thanks in advance.

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