Thanks for this bug report, but it’s an issue of mailbox.org. I have tested with a VCard with multiple numbers. As you can see, DAVdroid sends this VCard to the server which accepts it:
http-outgoing-20 >> "PUT /carddav/27/f99acddb-6020-4eb9-90e1-372b1f952e96.vcf HTTP/1.1[\r][\n]"
http-outgoing-20 >> "If-Match: http://www.open-xchange.com/carddav/f99acddb-6020-4eb9-90e1-372b1f952e96_1415042113090[\r][\n]"
http-outgoing-20 >> "Content-Type: text/vcard[\r][\n]"
http-outgoing-20 >> "Content-Length: 280[\r][\n]"
http-outgoing-20 >> "Host: dav.mailbox.org[\r][\n]"
http-outgoing-20 >> "Connection: Keep-Alive[\r][\n]"
http-outgoing-20 >> "User-Agent: DAVdroid/0.6.5[\r][\n]"
http-outgoing-20 >> "[\r][\n]"
http-outgoing-20 >> "BEGIN:VCARD[\r][\n]"
http-outgoing-20 >> "VERSION:3.0[\r][\n]"
http-outgoing-20 >> "REV:20141103T191513.090Z[\r][\n]"
http-outgoing-20 >> "UID:f99acddb-6020-4eb9-90e1-372b1f952e96[\r][\n]"
http-outgoing-20 >> "FN:Test User[\r][\n]"
http-outgoing-20 >> "N:User;Test;;;[\r][\n]"
http-outgoing-20 >> "TEL;TYPE=cell,pref:662453[\r][\n]"
http-outgoing-20 >> "TEL;TYPE=work:9675[\r][\n]"
http-outgoing-20 >> "TEL:68437[\r][\n]"
http-outgoing-20 >> "TEL;TYPE=x-custom:28866[\r][\n]"
http-outgoing-20 >> "PRODID:DAVdroid/0.6.5 (ez-vcard/0.9.6)[\r][\n]"
http-outgoing-20 >> "REV:20141103T191359Z[\r][\n]"
http-outgoing-20 >> "END:VCARD[\r][\n]"
http-outgoing-20 << "HTTP/1.1 201 Created[\r][\n]"
http-outgoing-20 << "Date: 2014-11-03T19:17:21Z[\r][\n]"
http-outgoing-20 << "Server: Openexchange WebDAV[\r][\n]"
http-outgoing-20 << "Content-Length: 0[\r][\n]"
http-outgoing-20 << "Strict-Transport-Security: max-age=15768000[\r][\n]"
http-outgoing-20 << "Keep-Alive: timeout=5, max=100[\r][\n]"
http-outgoing-20 << "Connection: Keep-Alive[\r][\n]"
http-outgoing-20 << "Content-Type: text/x-vcard[\r][\n]"
Immediately after having received the VCard, the server changes it:
http-outgoing-21 >> "GET /carddav/27/f99acddb-6020-4eb9-90e1-372b1f952e96.vcf HTTP/1.1[\r][\n]"
http-outgoing-21 >> "Accept: text/vcard[\r][\n]"
http-outgoing-21 >> "Host: dav.mailbox.org[\r][\n]"
http-outgoing-21 >> "Connection: Keep-Alive[\r][\n]"
http-outgoing-21 >> "User-Agent: DAVdroid/0.6.5[\r][\n]"
http-outgoing-21 >> "[\r][\n]"
http-outgoing-21 << "HTTP/1.1 200 OK[\r][\n]"
http-outgoing-21 << "Date: 2014-11-03T19:17:23Z[\r][\n]"
http-outgoing-21 << "Server: Openexchange WebDAV[\r][\n]"
http-outgoing-21 << "ETag: http://www.open-xchange.com/carddav/f99acddb-6020-4eb9-90e1-372b1f952e96_1415042241986[\r][\n]"
http-outgoing-21 << "Accept-Ranges: bytes[\r][\n]"
http-outgoing-21 << "Content-Type: text/vcard;charset=utf-8[\r][\n]"
http-outgoing-21 << "Content-Length: 180[\r][\n]"
http-outgoing-21 << "Strict-Transport-Security: max-age=15768000[\r][\n]"
http-outgoing-21 << "Keep-Alive: timeout=5, max=99[\r][\n]"
http-outgoing-21 << "Connection: Keep-Alive[\r][\n]"
http-outgoing-21 << "[\r][\n]"
http-outgoing-21 << "BEGIN:VCARD[\n]"
http-outgoing-21 << "VERSION:3.0[\n]"
http-outgoing-21 << "PRODID:OPEN-XCHANGE[\n]"
http-outgoing-21 << "FN:Test User[\n]"
http-outgoing-21 << "N:User;Test;;;[\n]"
http-outgoing-21 << "TEL;TYPE=cell,voice,pref:662453[\n]"
http-outgoing-21 << "REV:20141103T191721.986Z[\n]"
http-outgoing-21 << "UID:f99acddb-6020-4eb9-90e1-372b1f952e96[\n]"
http-outgoing-21 << "END:VCARD[\n]"
As you can see, only one number is left.
We will report this problem to mailbox.org, including a link to this issue.