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I have a snom320 and snom821 and on both phones I expected that the phones will ringing and ringing again  although another phone answer the call. Now I found in the settings the parameter ringing_time which is configured with a value of 120 seconds. 

In my scenario it makes no sence that the snom phone will ring 2 minutes although another device answer the call. What is the setting to stop ringing til the telephone system stop sending an incoming signal?

I changed the system settings regarding the ringing time to

<ringing_time perm="">0</ringing_time>

and now the 821 phone will ring every time 1 minute although the caller hang up.

Can someone tell me if the configuration item is correct and how to configure the phone, that every call stop ringing when the PBX system also stop ringing?