What you asking for is not a common request, so there is no report or tool you can run in the UCM GUI to get the information. But that does not mean you cannot get the information. One thing I want to point out is that the e.164 mask is set on a per device instance. So 2 devices with the same DN can have different masks. So when you address this problem in your environment, you may need to make the update on different devices (or use the 'update shared device settings')
I was able to create an SQL query that returns a list of the external mask per DN per phone. If you telnet to the your publisher UCM and run the following command:
run sql select d.name, d.description, n.dnorpattern, dnpm.e164mask from devicenumplanmap dnpm inner join device d on d.pkid = dnpm.fkdevice left outer join numplan n on n.pkid = dnpm.fknumplan
This will show ALL your DN's and their external masks, but you should be able to capture the output and sort to find which DN's on which devices need to be updated.
Please respond to let me know if this provides the information you are looking for.
Technical Marketing Engineer