A common workaround for NAT Issues is to change the SIP source port to a different number on each handset.
This needs to be done on the web interface for each phone.
The setting to be changed is Settings > SIP > Local SIP Port:
Each phone should be changed to a different number, we suggest starting at 5081 and increment upwards.
After changing the Local SIP port on all of your handsets, please test and confirm whether the issue is resolved.
If not, you may also need to reboot your router, followed by each handset.
See also: NAT Issues