Fails on frozen strings.

[beporter]

[1] pry(main)> Phoner::Phone.parse('+1-309-456-7890'.freeze)
FrozenError: can't modify frozen String: "+1-309-456-7890"
from /bundler/ruby/2.7.0/gems/phone-1.2.3/lib/phone.rb:170:in `sub!'

This is on version 1.2.0.

parse() could probably call string.dup to create its own internal mutable copy of the input.