My kyocera km-2550 multifunctional copier has started showing a error message "call for service c6000" and a phone number in asterisks. kindly advice on what to do or if there any online help.

This is fuser unit error code, you can try to reset it might work, but if the erorr code came back then you need to change the fuser unit.

In order to reset this code: follow those steps
on the panel press 1087 1087
The screen will change to service mode
on the panel again press 163
now look at the screen and you will see Excute.
Select Excute and press the Start key.
The Machine will rebot or it will asks you to turn it off and on.
After the machine load again, it should be good, if the Error came back!! Then change the Fuser Unit.

I’m happy to help further over the phone at https://www.6ya.com/expert/mustafa_06f84c65ab5cc745