Hi Narasimha,
Sometimes the outbound IP can be one that is not included in the list of possible outbound IPs. To combat this you can use VNet integration with Azure NAT Gateway so that you will have a static public IP:
Azure NAT Gateway integration
https://learn.microsoft.com/en-us/azure/app-service/networking/nat-gateway-integration
Below article provides information on how IP addresses work with App Service, when they change, etc. When reading, please keep in mind what I mentioned above regarding outbound IP addresses not listed in possibleOutboundIPaddresses being used, since it is key reason why I'm suggesting you use NAT Gateway.
Inbound and outbound IP addresses in Azure App Service
https://learn.microsoft.com/en-us/azure/app-service/overview-inbound-outbound-ips
Please click Accept Answer if the above is helpful. If something is unclear or you have questions, please add a comment below.
Thanks.
-TP