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Cisco VoIP phones not working properly with SonicWALL firewalls

BNMC Tech Tip

Question:  We have a new Voice over IP phone system (VoIP) with Cisco VoIP phones, a cloud hosted PBX and SonicWALL firewalls.  We have problems picking up a new line to start a call and we have dropped calls.  How do I fix this?


Answer:  The most likely scenario is that you have to set the Consistent NAT feature on the SonicWALL firewall to prevent this from happening.  Please refer to the KBA below on how to make this change.

SonicWALL Knowledge Base Article:


Used to resolve the following issues:

  • Busy Signal when trying to create a new call
  • Invalid line signal when trying to create a new call
  • Dropped calls internal and external
  • Dropped call during conference call

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