Getting a busy tone when transfer a call to extension

Hello, there are few factors need to consider:

- Dst. Extension is local:

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If the ext is local then it might not be using Firewall to route the traffic

In case there are multiple vlans, then we need to make sure all required SIP traffic is allowed from Vlan1 SRC to Vlan 2 DST

- Dst. Extension is Remote:

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If the DST is remote then we need to make sure both site devices have SIP ports allowed

Further you might need to make sure routing is correct, if its using VPN then desired subnets should be allowed on Phase2

You can also perform sniffing and debugs on the firewall to review traffic flow

https://community.fortinet.com/t5/FortiGate/Technical-Tip-Packet-capture-sniffer/ta-p/198313

I'm getting the same thing with our phone system.  Did you find any way to alter that audio?  The transferred call goes through as expected, but on the transferring side of things it does seem a bit odd to hear the busy signal.

I have the same issue that just started as well. Two weeks ago we did firmware updates on all forti equipment, phones and everything. No issues for a week. Then last week, after a power outage we started to have this problem. The transfer works but with a busy signal. Anyone find anything yet?

https://community.fortinet.com/t5/FortiGate/Technical-Tip-VoIP-and-SIP-configuration-and-troubleshooting/ta-p/342841