Problem
You want to have validations for phone numbers in Ruby on Rails code.

Solution
The mobile numbers in UK should start with a 0, followed by 7, then a number between 5 and 9, and finally another 8 numbers.
For the landline numbers in UK the number should start with 0, followed by either 1 and 8 or 9 digits, or by 2 or 3 and then 9 digits.
So in order to use a regular expression in your validations, you could use something like the following in your model:

case phone_no_type

  when 'mobile'

    if phone_no !~ /\A(([0][7][5-9])(\d{8}))\Z/

      errors.add('Mobile number")

    end

  when 'landline'

    if phone_no !~ /\A([0])((([1])(\d{8,9}))|(([2-3])(\d{9})))\Z/

      errors.add('Landline number')

    end

  end