Cisco Router VPN Details

show crypto isakmp peers           - Show ISAKMP peer structures (IPs and etc connected over the VPN)

show crypto isakmp sa                  - Show ISAKMP Security Associations (IPs and etc connected over the VPN)

show crypto session summary   - Show VPN groups and their members (users and groups connected over the VPN)

who                                          - Shows users currently logged into the router

SIP Carrier Debugs

SIP: Attribute mid, level 1 instance 1 not found.
2524735: Feb 11 17:54:53.522 MST: //-1/xxxxxxxxxxxx/SIP/Msg/ccsipDisplayMsg:
Received:
SIP/2.0 200 OK
Via: SIP/2.0/UDP >:5060;branch=z9hG4bK2D041732
Record-Route:
From: "Bob Jones" ;tag=BAAA8198-462
To: ;tag=_1255064163-933720387
Call-ID: 876EDB09-357911E0-970ADCDF-F107A873@>
CSeq: 101 INVITE
Contact: sip:+18004444444@67.231.0.94:5070
Accept: application/sdp, application/isup, application/dtmf, application/dtmf-relay,  multipart/mixed
Allow: INVITE,ACK,CANCEL,BYE,REGISTER,REFER,INFO,SUBSCRIBE,NOTIFY,PRACK,UPDATE,OPTIONS,MESSAGE,PUBLISH
Content-Disposition: session; handling=required
Content-Type: application/sdp
Content-Length: 276

v=0
o=pvp 305419896 305419896 IN IP4 67.231.0.125
s=-

! MEDIA GATEWAY - this is the carrier your SIP provider is having route your call. The IPs may change even if it's the same carrier but an IP whois lookup will show you which carrier your call is routing through. 
c=IN IP4 67.231.0.125

t=0 0
m=audio 24396 RTP/AVP 0 18 101
a=rtpmap:0 PCMU/8000
a=rtpmap:18 G729/8000
a=fmtp:18 annexb=no
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15
a=ptime:20
a=maxptime:40
a=sendrecv

Cisco Command Tricks - Monitoring Performance

Show the list of tasks running sorted by CPU usage and excluding any that are using 0%

sh proc cpu sorted | ex 0.00

Show spanning tree changes

show spanning-tree detail | inc ieee|occur|from|is exec

Show interface counter errors

sh interface counters errors 

sh int | i line|error

Show interface queue statuses and drops

sh int | in Input|line