There are lots of commands, it matters on what you want to check.
ISDN:
show isdn status
show isdn service
T1:
show controllers t1
show network-clock
DSP:
show voice dsp detail
show voice dsp group all
SIP:
show sip status reg
show sip reg stat
show sip service
Dial-plan:
show dial-peer voice summary
Active calls:
show voice call summary
show voice port summary
show call active voice brief
show voice call stat
Otherwise, check out this excellent link for more scenarios:
http://www.cisco.com/en/US/tech/tk1077/technologies_tech_note09186a0080207ec6.shtml
hth,
nick
