Assuming that you are in a country where caller ID is transmitted after the first ring, the delay should be about 8 seconds. If it is much longer, connect to the Asterisk console with sufficient verbosity and make a test call while watching it. If there is a long delay before seeing any action, perhaps
Also, try temporarily connecting one line to an analog phone (preferably line-powered corded) and see when it starts to ring, relative to what the caller hears.
Confirm that calling from one extension to another (via the PBX, not the DECT intercom mode) rings immediately.
If you still have trouble, post details about where the delays are occurring.