You should have been given the number when you bought premium support. If you don't have premium support, then there is no telephone support.
You will need to raise it as a support request over at https://support.atlassian.com/contact , but you might want to simply ask here as well - the community is here to help as well as talk.
Atlassian has a pretty good troubleshooting doc on Application Links.
High level things to check:
We’re excited to introduce external collaboration for Confluence, now available in early access. It is available to preview for Confluence Cloud Premium and Enterprise customers. (If you're not on ...
Connect with like-minded Atlassian users at free events near you!Find an event
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no Community Events near you at the moment.Host an event
You're one step closer to meeting fellow Atlassian users at your local event. Learn more about Community Events