M700 / M25 no calls possible after firmware upgrade

Hi community,

I have a Snom M700 base station with two M25 mobile phones. The base station is connected to an asterisk PBX server (16.28.0 from Debian repository). With the old firmware 324 all works fine. But I have to connect a new M30 mobile phone. For that functionality, I have to upgrade the firmware to 530B10. 

My steps are:

1. Update the firmware to 355 on M770 and M25 as described in  Wie man die M300,M400,M700,M900 DECT-Basisstation manuell aktualisiert
2. Update to 530B10 as described in BSV530B10 (base) HSV530B10 (handset) - Official Release 

The update process performed successfully, but the mobile phone can not call internal or external. A rustling noise is audible since that moment.

To solve that problem, I search on the internet and the Snom community. I found some interesting information on previous updates and some other topics that the update process needs additional steps with previous releases. 

In the update description, I found also the information that a downgrade is only possible to 450BXX. 

So I downgrade my M700 and the M25 mobile phones to 450B13 to solve the problem, but without success. The noise can no longer be heard, but calls are still not possible.

Witch update steps can I perform without crashing the M700 base station? Are there any suggestions to solve my problem?

Thanks for your reply.