Your description suggests some of the phone numbers have been input into the Excel workbook with the hyphens, whilst others have not. Consequently, you have a mix of numeric and non-numeric data. That, in turn causes the mailmerge to read the non-numeric data as formulae (i.e. the second part of the phone # gets subtracted from the first part). The solution is to remove all the hyphens in Excel.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]