I'm using an Blackberry Priv with Android 6.0.1 and sync my address book with mailbox.org.
Everything works fine, except if I mark an contact as favorite in the telefon app (gets marked with an star) and mark a number different than "privat" as standard, this information gets reset after every sync. I understand, that there is no field on the server to save this information,
For testing I've just changed 1 contact on the phone.
In the extended log I can see, that this contact is uploaded.
But now the contact on the server (without this information) is different than local and gets downloaded during the next sync and the standard telefon number is overwritten locally.
Is there any way to avoid this cycle?