No network-initiated dedicated bearer for VoLTE call?

[NathanA]

I am playing with different ROMs on MSM8994-based UE. For VoLTE, this phone uses Qualcomm IMS stack, not vendor-proprietary.

With stock vendor ROM, VoLTE works. However, if I replace vendor's custom Android with AOSP-based Android, while keeping vendor IMS binaries (ims services, ims.apk, ims .so libs, etc.), IMS registration works, but calls fail.

Note that in both cases, the same baseband/modem firmware is used, and modem configuration (EFS / NVs) is also completely identical.

I captured logs of calls from vendor Android vs. AOSP Android and analyzed in QCAT, and SIP INVITE and SDP contents are also 100% identical. The difference is that when calls fail, network responds to SIP INVITE with "503 Service Unavailable" and reason "INSUFFICIENT_BEARER_RESOURCES". This happens because network never initiates dedicated QCI1 bearer set-up to UE (RRC RECONFIGURATION never received by UE).

I cannot figure out why network is reacting differently when UE application processor firmware is different, especially since modem seems to abstract away & handle all aspects of IMS support autonomously from Android...so if modem firmware and EFS contents are exactly the same, why would network response change?

When default bearer for IMS PDN is first brought up, does UE need to do something to allow network to initiate dedicated bearer set-up in the future?

Thanks for any pointers...