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Based on our Support team experience and past tickets, there can be some issues when registering a desk phone with a Cisco product, or when making/receiving calls even if a phone figures as registered.
Typically, when using a Cisco Call Manager, this parameter needs to be enabled (it can be found on the phone WUI, Identity → SIP → Support Broken Registrar).
The following parameter can also help when changed and set to "CCM" (this can also be done from the WUI,  Identity → SIP → Server Type Support, and requires a reboot).
When registering a desk phone with a Cisco router instead, it can be useful to set the user_server_type to "Asterisk", since Cisco's routers are not working the same way as the Call Manager PBX.
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