Getting error no 98 when UDP multi socket is tried on an Ubuntu 20.04 x64 system

[Palezvar]

I have a fresh Ubuntu 20.04 LTS install and latest sipp.

I have this scenario that simulates extension to extension calling thru an IP-PBX: sipp [IP-PBX IP Address] -sf invite_with_register.xml -inf register_invite_client_list.csv -recv_timeout 500 -m 2 -l 2 -mp 10000 -t un -max_socket 10

I am getting the following error when I execute this test: Unable to bind UDP socket, errno = 98 (Address already in use)

Has anyone run into this issue? Please let me know if I need to provide more details about my scenario.

The scenario works for TCP protocol.

Thanks