Add Telephone to customer registration,customer edit page etc. run following setup script from magento root.
<?php
require_once('app/Mage.php');
umask(0);
Mage::app();
$installer = new Mage_Customer_Model_Entity_Setup('core_setup');
$installer->startSetup();
$vCustomerEntityType = $installer->getEntityTypeId('customer');
$vCustAttributeSetId = $installer->getDefaultAttributeSetId($vCustomerEntityType);
$vCustAttributeGroupId = $installer->getDefaultAttributeGroupId($vCustomerEntityType, $vCustAttributeSetId);
$installer->addAttribute('customer', 'cust_telephone', array(
'label' => 'Customer Telephone',
'input' => 'text',
'type' => 'varchar',
'forms' => array('customer_account_edit','customer_account_create','adminhtml_customer','checkout_register'),
'required' => 0,
'user_defined' => 1,
));
$installer->addAttributeToGroup($vCustomerEntityType, $vCustAttributeSetId, $vCustAttributeGroupId, 'mobile', 0);
$oAttribute = Mage::getSingleton('eav/config')->getAttribute('customer', 'cust_telephone');
$oAttribute->setData('used_in_forms', array('customer_account_edit','customer_account_create','adminhtml_customer','checkout_register'));
$oAttribute->save();
$installer->endSetup();
?>
Run this Script, it will add attribute "cust_telephone" and now you can use this attribute in the register form, like this
<li class="fields">
<div class="field">
<label><em>*</em><?php echo $this->__('Customer Telephone') ?></label>
<div class="input-box">
<input type="text" name="cust_telephone" value="<?php echo $this->escapeHtml($this->getFormData()->getCust_Telephone()) ?>" title="<?php echo $this->__('Customer Telephone') ?>" class="input-text" />
</div>
</div>
</li>
the code is tested, will work 100% ;)

