Dial timeout issue

Hi all,
I just installed a Polycom IP4000 for our office and run into an interesting problem. When I try to dial a 3 number extension directly from the dial tone, Asterisk only picks up the first 2 numbers. If I dial first and then take the phone off hook, everything works perfectly.

I know it’s a timeout issue and is probably quite simple to fix, but I can’t find what setting that is, nor where it should be set!
I’m guessing it’s in the phone’s config file in the tftp server. Is that correct? Help!