Is your server in the "sip.example.com" or is the host name "sip.example.com" there is some confusion as Fusion (and Asterisk) refer to the collection of phones and the server as a "domain" but my experience is that the term "domain" as used phone server world means the host name of the server. Type getting rid of your subdomain "sip.example.com"in DNS, and then put an A record in example.com "sip.example.com (with the right ip address), and purge the cache on the DNS server (or restart DNS) and I would restart phone server as well.