I’ve been in contact with the support staff of the secure messaging app Threema. Upon installation this app reads my address book, creates hashes from all telephone numbers and email addresses and compares these hashes to those stored on the Threema servers. If matches are found, the app then wants to write the Threema-ID into a new field.
For my DAVdroid adress book this is not working and after elaborating this with the Threema support, they think that it must be due to Threema not being able to create this new custom field.
Might this be a DAVdroid issue that could be fixed here?