Thanks for confirming that the call method from all the remote sites is the same and over the internet. This tends to verify that we should think the configuration issue will be on the remote site firewall.
It sounds like you don't have access to that device. What you could do on the central site first is make a successful call and capture the debug information suggested above. Then repeat the process with the unsuccessful site so we can compare what the communication differences are with what you have access to.
On the remote site what type of firewall is present?
Will the call work if the opposite site initiates the call?