You should have a phone number (required) field on the checkout even its guest checkout. So the customer can submit their phone number as well.
Event/Observer method:
app/code/core/company/module/etc/config.xml
<global>
<models>
<company_module>
<class>Company_Module_Model</class>
</company_module>
<events>
<sales_order_place_after>
<observers>
<add_phone_paypal_orders>
<type>singleton</type>
<class>company_module/observer</class>
<method>addPhonePaypalOrders</method>
</add_phone_paypal_orders>
</observers>
</sales_order_place_after>
</events>
</models>
</global>
app/code/core/company/module/Model/Observer.php
<?php
class Company_Module_Model_Observer
{
public function addPhonePaypalOrders($observer)
{
$order = $observer->getEvent()->getOrder();
$telephone = $order->getShippingAddress()->getTelephone();
if(!$telephone)
{
$order->getShippingAddress()->setTelephone(123456789);
try
{
$order->save();
}
catch (Exception $e)
{
Mage::logException($e);
}
}
}
}