Auto fallthrough, channel 'SI...

im new in asterisk .
i am using following package
os ubuntu
asterisk-1.6.0.15
dahdi-linux-complete-2.2.0.2+2.20
libpri-1.4.10.1

sip.conf
[general]
context=default
allowoverlap=no
bindport=5060
bindaddr=0.0.0.0
tcpenable= yes
tcpbindaddr=0.0.0.0
srvlookup= no

register => joahim1:******@192.168.1.101/davy1
register => davy1:******@192.168.1.100/jaohim1


[joahim1]
type=friend
secret=******
qualify= yes
nat=yes
host= 192.168.1.101
dtmfmode=rfc2833
canreinvite=no
username=joahim1
context= mehdy
allow= all

[davy1]
type=friend
secret=******
qualify=yes
nat=yes
host= 192.168.1.100
dtmfmode=rfc2833
canreinvite=no
username=davy1
context=mehdy
allow =all


extension.conf

[general]
static=yes
writeprotect=no

[globals]
CONSOLE=Console/dsp
IAXINFO=guest
TRUNK=DAHDI/G2

DAVY=SIP/davy1
TRUNKMSD=1

[mehdy]

exten => 101,1,Answer()
exten => 101,n,Background(demo-nogo)
exten => 101,n,WaitExten()

exten => 1,1,Dial(SIP/jaohn1,10)

exten => 2,1,Dial(${DAVY},10)



ion for 'joahim1@192.168.1.101' timed out, trying again (Attempt #86)
[Nov 8 14:24:14] NOTICE[3656]: chan_sip.c:9740 sip_reg_timeout: --Registration for 'joahim1@192.168.1.101' timed out, trying again(Attempt #86)
[Nov 8 14:24:34] NOTICE[3656]: chan_sip.c:9740 sip_reg_timeout: --Registration for 'davy1@192.168.1.100' timed out, trying again (Attempt#87)
[Nov 8 14:24:34] NOTICE[3656]: chan_sip.c:9740 sip_reg_timeout: --Registration for 'davy1@192.168.1.100' timed out, trying again (Attempt#87)
[Nov 8 14:24:34] NOTICE[3656]: chan_sip.c:9740 sip_reg_timeout: --Registration for 'joahim1@192.168.1.101' timed out, trying again(Attempt #87)
[Nov 8 14:24:34] NOTICE[3656]: chan_sip.c:9740 sip_reg_timeout: --Registration for 'joahim1@192.168.1.101' timed out, trying again(Attempt #87)



== Using SIP RTP CoS mark 5
[Nov 8 13:56:51] WARNING[3818]: app_dial.c:1499 dial_exec_full: Unable to create channel of type 'SIP' (cause 20 - Unknown)
[Nov 8 13:56:51] WARNING[3818]: app_dial.c:1499 dial_exec_full: Unable to create channel of type 'SIP' (cause 20 - Unknown)
== Everyone is busy/congested at this time (1:0/0/1)
-- Auto fallthrough, channel 'SIP/davy1-091efb50' status is 'CHANUNAVAI


please help me .