Digit discard instructions (DDI) remove parts of the dialed digit string before passing the number on to the adjacent system. A DDI removes a certain portion of the dialed string (called party). Access codes are typically used to make a phone call that will be routed to the PSTN. The PSTN switch does not expect the access code, so the access code must be stripped out of the called party number before sending the call to the carrier.
Digit stripping is configured in the Called Party Transformations section by selecting a Discard Digits setting from the drop-down menu. Discard digits can be configured at the route pattern and at the route group details level of the route list.
Please refer to the below link for more information
If the calling number appended or called number appended ?
What the error message you hear?
Please, share the #debug isdn q931 --if it is ISDN line
Thanks
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