The best way to start diagnosing the problem is to use the Dialed Number Analyzer tool. This tool is part of Communications Manager. You can find details on how to use it at:
This tool can be very useful but it is also very processor intensive so it is best to run it off hours.
Use this tool to analyze what happens when you enter the number directly and then with the pattern that has the transform mask. By comparing the two result you may be able to see why the call is not successful.