I have an unlocked iPhone 6 that I have purchased outside of the US. The model is compatible with US bandwidths, because I have tested international roaming (T Mobile) in the USA. Can I use my iPhone 6 to move to the Inner Circle BYOD plan?
What does the Inner Circle checker say about your IMEI?
A known problem with non-US iPhones here in the US is the inability to provision properly for voice and sms on CDMA. LTE does work if such phones activate.
There is no easy way to know in advance other than getting a GSX report or having Apple identify the current policy.
If the phone has an inappropriate activation policy, it might be possible to get Apple to change it but that can be a challenge.