Snom 220 - Manual (Page 59)

Settings
59
Publish Presence
Control the “presence” status information through this setting.
Refer-To Brackets
To switch additional brackets on or off in the Signaling for Refer-To.
Require PRACK
To force the usage of PRACK, choose “on” here.
Symmetrical RTP
In case you want to use symmetrical RTP, switch it “on” here.
Offer GRUU
This setting is used to toggle the support for GRUU (Globally Routable
User agent URL's) in SIP.
Offer MPO
Using this setting, the user can turn the Media Path Optimization on or
off.
Challenge for Dialog
Turning this setting on enables a challenge response for dialogue
messages.
Challenge for Reboot
This setting enables/disables a challenge response for remote reboot
request.
Challenge for Check-Sync
Turning this setting on enables a challenge response for Check-Sync.
UPDATE
Update Policy
You can select the best possible update policy you would like to adopt
for your phone.
Setting URL
Set up the URL of the settings server from where you would like to
obtain the configuration file to configure your phone.