Amongst others the asterisk server is connected via a BRI channel
(EuroISDN) to the carrier. It takes a very (noticeable) long time until an incoming call is established and the phones being connected to Asterisk start ringing.

As a test scenario I directly connected two different native BRI (ISDN)
phones to the delivery point (FXS port) of our carrier. I started a call from my cell phone and as soon as I finished typing the number and pressed the dial button of my cell phone, the native ISDN phone started to ring. There was no noticeable amount of time between the number being finished and the ringing.

If the Asterisk server is connected to the delivery point (FXS port), there is a noticeable gap (approx. between 5 and 15 seconds) after the number has been completed on my cell phone and before Asterisk starts to ring the connected VoIP phones. During this time a see the message “Connection establishing …” on the display of my cell phone. Sometimes the process even fails and I get a “Participant unavailable” message from my cell phone’s carrier.

If I enable “core set verbose ” on the Asterisk CLI, the message that there is an incoming call shows up at the end of this time gap. This means, the message on the Asterisk CLI and the ringing of the VoIP phones happens simultaneously. For this reason I guess this is not a problem of the dial plan but that the problem must be located on a lower level
(probably DAHDI), because Asterisk receives the event of an incoming call with delay.

Any ideas?