Hi Sandeep,
can you elaborate a bit on your setup? What deployment method did you use (binary, omnibus, docker, k8s, …)? Is there a firewall, loadbalancer or reverse proxy in front of your Mattermost server and if so, what does the Mattermost related configuration look like?
If you can establish a call but it drops after a few seconds this most likely is caused by either connection timeouts or improper NAT/tunneling configurations on devices in front of the Mattermost application server.