Dear VoLTE Experts.
During SRVCC to UTRAN test, we observed that if the UE make SRVCC to UTRAN before the called number answer (during ringing phase), the call will be disconnected.
Anyone faced this issue before?
Do you have LAC TAC mapping with respect to 2G or 3G?
Does any location or routing area update happened after disconnect?
To 3G.
Yes.
If SRVCC happened after the called number answer no issue at all.
If LAC TAC mapping is same with 3G, then why is it happening?
Don’t know. 
Is there a reason in the disconnect message?
This happens when SRVCC handover at call setup in pre-alerting phase is not supported.
I am not sure about what is you Core and RAN Vendor. But do check SRVCC Delay Timer.
This will delay the SRVCC handover to GSM/WCDMA in the pre-alerting phase and triggers it only after SIP 180 (Ringing) response.
This will save the VoLTE call being dropped.
SRVCC delay time from Core or RAN side?
RAN. But there must be something in parallel in Core too.
Thanks.
Do you know the RAN parameter?
Vendor is Huawei.
SRVCC Delay Timer or SRVCC Wait Timer (Request to Receive Response).
For Ericsson I am aware it is srvccDelayTimer.
.
Update here:
Issue is related to IMS, we opened E2E trace, and Core team still checking.
OK, great.
It looks Core issue in our first discussion, secondly also do share the solution as well and findings.
Sure, will share after Core team give a solution, if not we will do the agreed actions.