function ChangePhonePrefix() {
	CountrySelect = document.getElementById('CountrySelect');
	PhoneNumberInput = document.getElementById('PhoneNumber');

	if (PhoneNumberInput.value=="" || PhoneNumberInput.value=="+420" || PhoneNumberInput.value=="+421") {
		if (CountrySelect.value=="1") {
			PhoneNumberInput.value="+420";
		} else if (CountrySelect.value=="2") {
			PhoneNumberInput.value="+421";
		}
	}
	return true;
}