asterisk users - Feb 2011 - Reliably getting sip extension name from channel variables

Hi

We're using asterisk 1.4.17 debian package soon moving to 1.8 rpm
package.

When using MixMonitor to do call recordings, for outbound calls I have
been using the channel variable SIPURI to get the originating SIP
extension name. I have now stumbled across a few files where the SIP
extension name must be incorrect when cross referencing the call with
other sources (such as the channel shown in the cdr).

So, a couple of questions I'm throwing out there:
Why would the Channel variable have a different SIP extension as part of
it's string to the same channels SIPURI variable?

Out of these variables which would you consider the most reliable one to
extract the sip extension name from:
CHANNEL
SIPURI
CALLERID(num) (please remember that this is for outgoing calls only)?

Thanks in advance

Ish
-- 
Ishfaq Malik
Software Developer
PackNet Ltd

Office:   0161 660 3062