I've already posted how to enable telnet and get to the shell for ShoreTel (non-V) switches here.
Here are some more advanced commands that I have found useful for troubleshooting:
Show network interface information:
Test LSP UDP connectivity to "192.168.1.1", repeat 100 times:
Show connectivity to other switches/servers:
Show all extensions known to switch:
Show detailed info for local extensions:
Show detailed info for remote extensions:
Restart the switch:
Show the routing table:
Show detailed SIP info on the console. sipdebuglevel=0 is the default
Note: Commands are case sensitive
More commands can be found in the ShoreTel release maintenance guides.