VoLTE block call due to SIP 500 Internal server error

Hi Experts.

Need to know the details of VoLTE block call due to SIP 500 Internal server error.

Call was initiated in good radio environment and then after this error UE performed CSFB and went to 3G.
What can be the possible cause?

Initial Findings: It may be possible that UE got deregistered from IMs server so thats why this issue happen.

Any opinion?

It’s built-in mechanism to setup call on legacy networks if Volte call fails. This is controlled by cscf functions at IMS.
Basically there is mechanism at CSCF to control traffic to be sent to destinations that has been blacklisted as unreachable due to SIP transaction timeout. For a specific time, CSCF rejects the traffic by sending 500 error to already determined proportion of traffic.
So the call isn’t blocked in Volte if csfb is successful.
Can you confirm if the call is blocked in legacy network with some error code like error 41?
If so issue is at higher levels and you need to setup IMS and access end volte traces where these scenarios are most frequent.