Whatsapp is an application that is uses by a lot of people to communicate. Allthough Whatsapp-messages look like SMS-messages, the way they are handled are different. They are stored in a database on the mobile phone with their own structure. Deleted Whatsapp-messages often can be retrieved from a mobile phone.
It is also possible to retrieve Whatsapp data from iTunes-backups or iCloud-backups, but best is to create a dump of the internal memory (and SD-card if one is in the phone).
Digirec has specialised forensic equipement to create a forensically sound copy of the data from the mobile device to be able to retrieve deleted Whatsapp messages.
Encase searchterm to find Whatsapp data on an iPhone:
Grep must be activated!

The above keywords wil overlap each others extractions, but give good results in finding most (parts of) whatsapp messages.