Any contact created on an Android smartphone needs to have a default phone number specified before it can be synchronized with your address book. If additional numbers are added to a contact using the smartphone, these will be stored locally at first but not necessarily synchronized with our servers if you use CardDav.
If users then add additional phone numbers to a contact in the address book using the web interface of mailbox.org, any further phone numbers added previously on the phone but not synchronized with our servers may be overwritten during the next CardDav sync.
Please note: In this case, any additional phone numbers previously added using the smartphone will be lost!
It appears that the reason for this malfunction lies in differing implementations of the CardDav standard, in particular with respect to the permitted amount of phone numbers per entry. We have notified the software manufacturer and are waiting for a solution. Unfortunately, we have not been given yet any indication of the release date of a corresponding patch that would fix the issue. Of course, we are quite unhappy with the situation and will keep up the pressure to get this problem resolved as soon as possible.