test16.rb

require 'active_record'

class Person < ActiveRecord::Base
  establish_connection adapter: 'sqlite3', database: 'foobar.db'
  connection.create_table table_name, force: true do |t|
    t.string :mobile_no
  end

  validate do
    regexp = /^(07[\d]{9})/
    unless mobile_no.match(regexp)
      errors.add(:base, 'has wrong format')
    end
  end
end

bob = Person.new(mobile_no: '07000000000')
p bob.valid?
p bob.errors.to_a
john = Person.new(mobile_no: 'wrong')
p john.valid?
p john.errors.to_a

# > ruby test16.rb
# true
# []
# false
# ["has wrong format"]