Hi Paul,

I have found it in this link - https://github.com/zephyrproject-rtos/zephyr/issues/10561
By referring the config file also we can able to find Echo client and server example can be built for either OpenThread or third party IP stack.
https://github.com/zephyrproject-rtos/zephyr/blob/master/samples/net/echo_client/overlay-ot.conf
Remaining config files will be used to build for other configurations -  https://github.com/zephyrproject-rtos/zephyr/blob/master/samples/net/echo_client/

Thanks & Regards,
Deepa