PCAP logging on Fortigate 40c

One of my clients has an older Fortigate 40c that we are trying to figure out how to do a PCAP log on in order to troubleshoot their Fortivoice phone system.  Support told me to run the command,  but it comes back with a "get time error."

diag sniffer packet any 'host <Foritoivce ip address>' 6 0 l 

Is there another command I should try to run, or something else I can try?  Unfortunately I'm a novice at this type of thing on the Fortigates.

Thanks.