asterisk users - Jul 2010 - sip peer becomes unreachable in Asterisk 1.6

Hello,

I recently upgraded from asterisk 1.4 to 1.6. I am using the same SIP
settings in sip.conf in this version also. I am facing  a problem when
a SIP client makes a call.

When a SIP client registers to asterisk its status shows 'OK' and it
is able to receive incoming calls. But as soon as this client make a
call, its status becomes 'UNREACHABLE' and it cannot receive any
incoming calls. Its status remains 'UNREACHABLE' until it re-registers
again. I have faced this problem on various versions of 1.6 (1.6.2.0,
1.6.2.7, 1.6.1.1), but this never happened in the 1.4 (1.4.24, 1.4.26)
versions.

I have kept the SIP re-register time in the clients to a very small
value to avoid becoming 'UNREACHABLE' for a long time, but his
doesn't
seem to be the solution. Is there any specific SIP settings which
needs to be made in 1.6 to avoid this problem?

I am using realtime sip. Some of my sip settings are
rtcachefriends=yes
rtupdate=no
qualify=yes
canreinvite=yes
nat=yes

Thanks