Because you mention it's a C Series, I assume you're referring to the C20, and not E20, which are two different products.
Can you tell us more about how you have the endpoint configured and deployed?
Just from what you describe, you're getting that message because the endpoint is configured to use a Gatekeeper such as a VCS to make/receive calls. If you do not have a Gatekeeper, you can deploy the C20 as a standalone endpoint though, example configuration of this can be seen in thread sx20-standalone.
You should also consider upgrading the software to TC7, as TC4 is way outdated and no longer supported.
