for skype you will need its own ruleset to allow skype through your proxy, like those example below.
In case you still have issues please check rule engine trace for possible blocks and authentication issues. Another possible root cause could be an outdated skype version.
Can you please check if you have the same issue with older Skype versions? There are known issues for current Skype version in the call functions with (any) proxy.
at the moment we have the same issue with different Skype version (tested 188.8.131.52 and 184.108.40.206) do you have any suggestion to resolve this issue?
unfortunately I have not had a chance to try the suggestions of serjei...
I am aware of version 7.15 is working, if only the call functions are not working with the newer version, I assume that this is the issue of the Skype version and we cannot do anything on Proxy side.
As Skype has been working for you previously, I assume that the rulesets are correct.