asterisk users - Dec 2014 - Slow (Lagging) connection establishment via BRI

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 <number>" 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?
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