Essentially, I want to be able to do something like the following:
$session = Mage::getSingleton('customer/session');
$customer = $session->getCustomer();
$customer->setTelephone($telephone);
$customer->save();
And such as this:
$session = Mage::getSingleton('customer/session');
$customer = $session->getCustomer();
$response["telephone"] = $customer->getTelephone();
Without having to add a default billing/shipping address, since the customer will need to enter his mobile number on registration, but not a billing/shipping address.
Is that possible?