asterisk users - Aug 2011 - Timer B in sip.conf cannot be changed

I am using 1.8. I need to change timerb to 6500, that is, if there is
no response of some sort in 6.5 seconds, consider the call failed and
try another route. It does not matter what do I set for the other
timers:
T1min=100
timert1=100
Timerb=6500

The command "sip show settings" always shows Timer B=32000. Any ideas
how can I reduce Timer B?