We recently upgraded out AjaxControlToolkit from version 4.5.7.123 to 15.1 and it appears to have introduced a bug where a RegularExpressionValidator will say the input is invalid when a MaskedEditExtender is associated with the same TextBox control as the RegularExpressionValdiator. We reverted to the old version and this was no longer an issue. Our markup and values for the regular expressions (which are stored in Resource files) are included in the attachment.
The data we tested with for the Work Phone was 888-888-8888.