If not properly configured, your phones may exhibit the following symptoms:


Randomly fail to ring for incoming calls, though you can call out.


Calls go straight to voicemail.


Your main number ring group goes right to an auto-attendant or voicemail instead of ringing.


To resolve this, there are two settings you should make on your router, shown in the image below.  You should also have your router at a current or recent firmware version.  See the router documentation for firmware update instructions.