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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