def mobile_prefix(leading_zero = true)
      mobile_prefix = '1' + rand(5..7).to_s + rand(0..9).to_s
      mobile_prefix = '0' + mobile_prefix if leading_zero
      mobile_prefix
    end
def mobile_prefix(leading_zero = true)
      mobile_prefix = '1' + rand(5..7).to_s + rand(0..9).to_s
      mobile_prefix = '0' + mobile_prefix if leading_zero
      mobile_prefix
    end
Mobile prefixes are in the 015x, 016x, 017x ranges