MySQL query for telephone number lookup from Magento db

I use both Magento and an Asterisk pbx. Asterisk has a facility to lookup phone numbers in an mysql db. I want to lookup callers phone numbers in a magento mysql db and return their most recent order number. All I need is the appropriate sql command that will return this. It will need to make the appropriate joins and return results quickly. This link illustrates what I am trying to do, but we want to use Magento as our data source. [login to view URL]