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, or certain phones fail to ring at random.
To resolve this, should configure the "VoIP" section on your router settings as 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.
We also strongly suggest that you enable QoS to assure call quality. See this Sonicwall article: https://www.sonicwall.com/en-us/support/knowledge-base/170520190748385