About 2,800 6921 IP phones were using firmware 9.1.1.
To avoid a massive upgrade to firmware version 9.2.1 that comes with CUCM 8.6.2, it was installed firmware version 9.1.1 on the TFTP and this phone load was configured as default in Device Defaults section for 6921 models.
After the migration, we proceed to update gradually the 6921 ip phones to version 9.2.1
It was found that the parameters of Maximum Number of Calls and Busy Trigger can not be changed even though the phones already have the firmware version 9.2.1.
Maximum Number of Calls and Busy Triggers per Line are:
Maximum Number of Calls 2
Busy Trigger 1
It was installed a higher version, the 220.127.116.11 in the TFTP and then chose some 6921 IP phones to be upgraded to this new one firmware. The parameters of Maximum Number of Calls and Busy Trigger are still unable to be changed.
The TFTP service has already restarted, without any change.