iPhone/iPod touch/iPad Contacts have fields for Custom and Other phone numbers and for Ringtone.
If used, the device does not send these fields to the server. The contact will be missing these fields when viewed on the server.
In addition, if a change is made to the contact on the server, it is synchronized back to the device.
Since the contact change coming from the server does not contain the custom/other phone number fields, they are deleted on the device when the change is synchronized. Ringtone, however, is preserved on the device.
On devices running iPhone OS 3.0, the Custom and Other phone number fields are not exposed when creating a new contact in an Exchange account. Since the fields are never created, deletions will not be an issue. Contacts created prior to an upgrade to OS 3.0, however, can still potentially be affected.