This happens as the number lookup process used by call routing does not find one explicitly match, so it waits for the inter digit timeout, which by default is set to 15 seconds. If you truly want to correct this you’ll need to make sure that you don’t have any overlapping matches in your dial plan. To lower the time waiting for the call to proceed you can lower the T302 timer in the CM service parameters.
This affects any calls, it is not limited to only external calls. If there is an overlap in match this will result in what your describe. If you do a DNA you’ll see what the system matches and if it has any alternate match(es). If it has you’ll get the wait for inter digit timeout as the call routing process cannot make an explicit match.
It could as well be caused by that you have a variable length number plan. For example you have extensions that are 1XXX and 1XXXX or +4681234XX and +4681234XXX. Both of these, or any similar permutation, are a bad thing as it requires CM to wait until the T302 timer expires before the call is routed.
You probably have a conflict dial plan which causing this issue when dialling extension.
use Dialed number analysis(DNA), you can see the alternative dial plan. Access the DNA from CUCM serviceability page.
when we says conflict/overlapping dial plan, assume that your extension are in 1xxx but you have route pattern 1.xxxxxxx. in this case if you dial 1022 the cucm will wait interdigit timeout to end. .
