It is simple to retrieve by using a data recovery software. I happen to know a freeware which may help you, download it here: http/freeware-recovery.blogspot.com/2012/07/free-delete-format-recovery.html
It's free of charge so why not have a try? Connect your cell phone to your pc and launch the software, select "Deep Scan Recovery" or "Advanced Files Recovery", then let the software scan your cell phone to find the deleted number. It is quite likely that it may find out many data, just preview and save the one you need. The operation is easy. Hope it works.