Try this:
Using cell A1
Set the numbe format to Special and select Phone Number
Then:
Data>Data Validation
Allow: Custom
Formula: =ISERROR(FIND("-",A1))
If length is an issue:
Use this validation formula instead:
=AND(LEN(A1)=10,ISERROR(FIND("-",A1)))
Does that give you something to work with?
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Regards,
Ron