This works, providing the alert plugin is enabled for the user, the channel is correct ([SIP/xxx] header in FOP2 autobuttons configuration ouput):
exten => 2222,n,UserEvent(Alert,Channel: SIP/111,Family: alert,Value: hello world~error~10)
It is possible that you will have to conver the channel to upper case, try setting it to SIP/21_SNOM in the user event and see if it makes a difference. IF you want to broadcast to all connected users, try the pseudo channel GLOBAL_ALERT.
Best regards,