phone_mobile missing in oder_conf e-mail

[jordiob]

I'm using one page checkout and everything works perfect except for the phone_mobile field, which is missing on the order_conf.html email once the order is placed.

All the others fields are arriving correctly and phone_mobile is displayed in order-detail, invoked by
{$address_invoice->phone_mobile|escape:'htmlall':'UTF-8'}

Anyone knows what can be the problem?

{delivery_phone_mobile} returns nothing
{invoice_phone_mobile} returns nothing
{phone} returns nothing
{phone_mobile} returns nothing