Today I signed up to an online number and monthly unlimited subcription.
When someone rings my online number I can accept the call in either the iOS app or my desktop app, but rather than forwarding to the mobile number I have nominated when Im not on either app, it is just going to the Skype voicemail service.
it is very possible that the number you are trying to forward to is not actually covered by your subscription (and presumably you don't have the required Skype Credit either)
have you tried using Skype to call that number? does it work?
in any event, your question has nothing to do with Skype Numbers, which is what this forum is about