Hi Dimitris,

Thanks for reaching out to us. This would require a custom validation script. Here is the one that you can use 

<script>
window.ameliaActions = {
  customValidation: function (success = null, error = null, data) {
    console.log('customValidation HOOK')
    console.log('------------')
    console.log(data)
    const customValidator = (rule, value, callback) => {
      let pattern = /^\d{10}$/;
 
      if (!pattern.test(value.replace(/\D/g,''))) {
        callback(new Error())
      } else {
        callback()
      }
    }
    data.rules.phone.push({required: false, message: 'The phone number must be 8 digits long', validator: customValidator, trigger: 'submit'})
  }
}
</script>

Custom code needs to be applied to the page where the booking form is. Depending on what you're using (Gutenberg blocks, or some page builder), adding the CSS or JS can be done in a few different ways. 

If you need help with adding custom CSS to the page, please take a look at this article.

If you need help with adding custom JS to the page, please take a look at this article.