$("#call-me-back-form").submit(function(event){
  var url = 'https://ccusage.yourservices.co.uk/click2dial/click2dial.php?sref=112927335Sk';
  event.preventDefault(); 
  var phone_number = $(this).find('#callback_number_field').val();
  if (!checkUKTelephone(phone_number)) {
    alert(telNumberErrors[telNumberErrorNo]);
  } else {
    $(this).find('#callback_number_field').val( phone_number );
    $.post(url, { destination:phone_number } );
  }
});

/*==============================================================================

The definition of a valid telephone number has been taken from:

http://www.ofcom.org.uk/telecoms/ioi/numbers/numplan030809.pdf

All inappropriate telephone numbers are disallowed (e.g. premium lines, sex 
lines, radio-paging services etc.)

Author:    John Gardner
Date:      16th November 2003

Version:   V1.1  4th August 2006       
					 Updated to include 03 numbers being added by Ofcom in early 2007.
					
Based on: http://www.braemoor.co.uk/software/telnumbers.shtml						
------------------------------------------------------------------------------*/

function checkUKTelephone (telephoneNumber) {

  // Convert into a string and check that we were provided with something
  var telnum = telephoneNumber + " ";
  if (telnum.length == 1)  {
     telNumberErrorNo = 1;
     return false
  }
  telnum.length = telnum.length - 1;
  
  // Don't allow country codes to be included (assumes a leading "+")
  var exp = /^(\+)[\s]*(.*)$/;
  if (exp.test(telnum) == true) {
     telNumberErrorNo = 2;
     return false;
  }
  
  // Remove spaces from the telephone number to help validation
  while (telnum.indexOf(" ")!= -1)  {
    telnum = telnum.slice (0,telnum.indexOf(" ")) + telnum.slice (telnum.indexOf(" ")+1)
  }
  
  // Remove hyphens from the telephone number to help validation
  while (telnum.indexOf("-")!= -1)  {
    telnum = telnum.slice (0,telnum.indexOf("-")) + telnum.slice (telnum.indexOf("-")+1)
  }  
  
  // Now check that all the characters are digits
  exp = /^[0-9]{10,11}$/;
  if (exp.test(telnum) != true) {
     telNumberErrorNo = 3;
     return false;
  }
  
  // Now check that the first digit is 0
  exp = /^0[0-9]{9,10}$/;
  if (exp.test(telnum) != true) {
     telNumberErrorNo = 4;
     return false;
  }
	
	// Disallow numbers allocated for dramas.
	 
  // Array holds the regular expressions for the drama telephone numbers
  var tnexp = new Array ();
	tnexp.push (/^(0113|0114|0115|0116|0117|0118|0121|0131|0141|0151|0161)(4960)[0-9]{3}$/);
	tnexp.push (/^02079460[0-9]{3}$/);
	tnexp.push (/^01914980[0-9]{3}$/);
	tnexp.push (/^02890180[0-9]{3}$/);
	tnexp.push (/^02920180[0-9]{3}$/);
	tnexp.push (/^01632960[0-9]{3}$/);
	tnexp.push (/^07700900[0-9]{3}$/);
	tnexp.push (/^08081570[0-9]{3}$/);
	tnexp.push (/^09098790[0-9]{3}$/);
	tnexp.push (/^03069990[0-9]{3}$/);
	
	for (var i=0; i<tnexp.length; i++) {
    if ( tnexp[i].test(telnum) ) {
      telNumberErrorNo = 5;
      return false;
    }
	}
  
  // Finally check that the telephone number is appropriate.
  exp = (/^(01|02|03|05|070|071|072|073|074|075|07624|077|078|079)[0-9]+$/);
	if (exp.test(telnum) != true) {
     telNumberErrorNo = 5;
     return false;
  }
  
  // Telephone number seems to be valid - return the stripped telehone number  
  return telnum;
}

var telNumberErrorNo = 0;
var telNumberErrors = new Array ();
telNumberErrors[0] = "Valid UK telephone number.";
telNumberErrors[1] = "Telephone number not provided.";
telNumberErrors[2] = "UK telephone number without the country code, please.";
telNumberErrors[3] = "UK telephone numbers should contain 10 or 11 digits.";
telNumberErrors[4] = "The telephone number should start with a 0.";
telNumberErrors[5] = "The telephone number is either invalid or inappropriate. Please ensure this is a landline or mobile phone number.";