Make sure that your phone number field is of type text.
When I test this in the data viewer:
Patterncount ( "555-5555" ; " - " )
Patterncount ( "555 - 5555" ; " - " )
Well, I checked that I was doing it right (field defined as text, phone numbers 'tight' - no space before or after the hyphen) and it still failed. I ended up substituting 'zzz' for ' - ' and then doing the pattern count on zzz to get the desired result. I have no idea what is/was going on.
I ran additional tests using both text and number type fields, testing both " - " and "-".
Each test produced the expected results.
I ran these tests using Windows 7, FileMaker 13 and English indexing.