Most likely, the UCx system is not configured to recognize the dialed number as a complete number that is to be dialed automatically and the dial out timeout is not enabled. If that's the case, the user would have to press # to initiate the dialing.
Go to PBX Configuration - Nortel Settings and review Dialplan Patterns in the section Dialing Options.
To enable the dial out timeout, add the pattern .T (that means any digit followed by the timeout period - the duration is configured above that section in the field Auto Dial Timer).
To configure automatic dialing of the phone number dialed by the user, add a corresponding pattern. For example, if the user dialed a seven digit local number 7771234, you could add the pattern 7xxxxxx to have the number automatically dialed after the 7th digit is pressed.
If you're in North America, you can add common NA patterns using the Dialplan Patters Wizards control below Dialplan Patterns.
A word of caution - do not use patters that are too generic. For example, a pattern such as [2-9]xx would make the system automatically dial any number that starts with 2 to 9 and has three digits - the pattern would therefore disallow dialing of longer numbers (such as the example 7771234).