In general you set this on the phone (Number Map) something like that.
What that does it checks how many digits the user has entered and dials directly after that.
So if i set my number map or number mask or what ever it is called to be 3 digits. The phone will burst after I entered 3 digits.
On the other hand if I set it up to be 10 digits and only dial 3 it waits for a few seconds to make sure that the user is not entering more digits.
If you provide your phone details and a link to the manual Ill have a look when I get back in the office.