Some of my calls going from 1 extension to another simple remote phone extension will go straight to voicemail of the simple remote phone. The phone is not in use and is still registered and some calls will work fine. The simple remote phone can always make calls to other extensions and outside calls.
The symptoms you are describing sound like your registration expire times are set for too long.
If a softphone is being used as a simple remote phone, you should set the softphone’s maximum registration request to a
time less than your firewall's UDP session timeout (for example, 55 seconds).
This can be done from the Phones GUI.
For Polycom: Login>Settings>SIP>Server1 and/or 2>Expires (s)>Change to 55
Also do the following: Settings>Lines>Server1 and/or 2>Expires (s)>Change to 55
See document: Configuring Simple Remote Phones for the NetVanta 7000 Series for more info.
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