From Snom User Wiki
snom phones using the following firmware images are pre-configured to interoperate with Nortel PBXs for german ITSP ARCOR:
Firmware image (based on version 6) download links :
Firmware image download links for Nortel CS2000 (Communication Server 2000) Release SN09:
The firmware features the following settings by default:
ARCOR Style WUI/ ARCOR login interface.. german language menus Identity 1: Displayname = SIP-Account = Rufnummer Support Broken Registrar: ON Long SIP-Contact (RFC3840): OFF RTP Encryption: OFF Refer-To Brackets: ON
You may also use any other firmware images as far as you set the above settings.