However, the column is a varchar(255) charset UTF8.
This is not okay.
In MySQL, that will take up 765 bytes per row in memory, plus will be doing UTF8 to to 3byte expansion for every row read or scanned. And it has to use the wchar string ops to do comparisons.
The ISO standard for PSTN phone numbers, E.164, states that phone numbers are a max of 15 digits long, and that includes the country code.
Make phone numbers a varchar(16) charset ASCII. Have your application mash out all the spaces and hypens.
Then marshalling and expansion is free, and modern processors can do an equality test against 16 bytes in a single operation.
16 bytes vs 765, that's a 50x savings.
Oh, and stop assuming that all phone numbers are in the US and Canada, and thus are of the form (AAA) EEE-TTTT. There are acceptably good library routines in your language of choice that will accept a string of digits, look at the country code, know that country's dialing plan, and output a "pretty formated" phone number. Don't try to keep pretty formated phone numbers in your database, and don't try to make the database server do the pretty formatting.