CUCM External Phone Number Masks

We are going to learn the concept of external phone number masks in CUCM and configure it.

On the private network (the IP Telephony network owned by the company), there is no display issues for the calling party. We control that by setting the Alerting name and the Internal Caller ID. However, when the call leaves the private network, the PSTN has a different view. The Alerting Name and Internal Caller ID fields become useless. With QSIG and DMS versions of PRI, the identities of the call parties (name, number) are transmitted to the PSTN network. But in modern PSTN networks and with today’s voice gateways, we need to carefully define how the calling number is presented to the PSTN. An exception to applying dialing transformations is when users are using fully qualified PSTN numbers between each others.
So usually, we need calling party transformations for display purposes across an external network. And here comes the External Phone Number Mask.

the External Phone Number Mask (or as I call it EPNM, for simplicity) allows a line to present its external number to the PSTN.
EPNM can be leveraged in auto-registration if the company dial plan is not overlapping and the company is confined to a single area code and office code. In that case, in auto-registration settings, we can add the following mask for example: 212555XXXX, where XXXX will be filled with the internal DN.
EPNM can also be leveraged to hide the external number of the line and substitute it with the company switchbord number. For example, setting the EPNM to 81105555 will make all external calls appear to have 81105555 as the calling number.
When a bestMatch route pattern has the “Use External Phone Number Mask”, the calling line will be presented to PSTN with its external number.

External phone number mask configuration

The External Phone Number Mask is not a directory number (DN) feature. It is rather a line feature. The proof is that it does not appear under the Directory Number Configuration. It appears however under the line configuration, or when you click on Edit Line Appearance in the Directory Number page, to edit the line settings.


Cover photo credit: Andreas Praefcke (Own work (own photograph)) [GFDL ( or CC BY 3.0 (], via Wikimedia Commons

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