Provisioning my ASUS Zenfone 4 Pro for VoLTE

I know for a fact that my ASUS Zenfone 4 Pro supports VoLTE.

It's not so much what people don't know that's the problem, it's what they know that isn't true.

Unless AT&T has certified your phone for AT&T's VoLTE implementation, it will not work on AT&T's network.  Will.  Not.  Work.  Below are two lists of phones that AT&T publishes.  The first are AT&T Branded model variants, the second is for "unlocked" phones that are supported.  Your phone appears on neither.  The issue has nothing to do with SIM provisioning.

https://www.att.com/idpassets/images/support/wireless/Devices-Working-on-ATT-Network.pdf

https://www.att.com/idpassets/images/support/wireless/Service-Capabilities-Unlocked-Devices-ATT-Network.pdf

So you are saying that even though the HW supports it [4G LTE + VOLTE], the FW (ROM) supports it, the OS supports it, so the whole device supports it,  AT&T will still deny the phone access just because the model is not matching an entry in a database list ?

Yep.  

[btw Indian carriers allow it]

So what?

So you are saying that even though the HW supports it [4G LTE + VOLTE], the FW (ROM) supports it, the OS supports it, so the whole device supports it,

To be fair to AT&T, this "database list" is not completely arbitrary.

Just because the manufacturer says the device supports VoLTE, and the device is supported on VoLTE on other carriers (whether or not they are in India), that doesn't mean that the phone has the codex that AT&T uses for VoLTE, and has been tested and found to work on the AT&T network.  

My understanding is the process to get on the list is for the manufacturer to submit the unit for testing, and the test must pass.  It may be that the manufacturer just declined to submit the unit for testing; this testing is not free.  It may be that the manufacturer doesn't want to license the codex that AT&T requires.  It may be that the phone failed testing.  Anyway, once the phone is accepted, the manufacturer then passes a list of IDs for those phones to be whitelisted as working.