Hi Ali,
On this page you don't add + , on this page use this format:
1-408-XXX-XXXX
but when you do a dialout via WebEx to PSTN phones that time call will come to CUCM as +Called Phone number so you need to have translation patter in CUCM to take off that leading '+' and send call without '+' to your PSTN provider.
HTH
Arun