Problem with my simplest setup of asterisk with OpenBTS

Problem Statement:
We are doing a project in which we are using both asterisk and OpenBTS. Everything works fine when both Asterisk and OpenBTS are running on the same machine. But, when I run Asterisk and OpenBTS on separate machines, I don’t understand what goes wrong. It says,
– SIP/IMSI410018230446233-00000020 is circuit-busy

Configurations:
My Configuration files are:

sip.conf

[general]
autokill=yes

[IMSI410035101450065] ;put my IMSI number
callerid=aqsa <6001>
canreinvite=no
type=friend
host=dynamic
;nat=yes
;secret=pass ; put a strong, unique password here instead
context=users
allow=ulaw
;deny=0.0.0.0/0
allow=gsm
;permit=10.99.18.155/255.255.255.0 ; replace with your network settings
;insecure=invite,port

[IMSI410018230446233]
callerid=ainee <6002>
canreinvite=no
type=friend
host=dynamic
;nat=yes
;secret=pass ; put a strong, unique password here instead
context=users
allow=ulaw
;deny=0.0.0.0/0
allow=gsm
;permit=10.99.19.89/255.255.255.0 ; replace with your network settings
;insecure=invite,port

[IMSI410070380139067]
callerid=sidra <6003>
canreinvite=no
type=friend
host=dynamic
;nat=yes
;secret=pass ; put a strong, unique password here instead
context=users
allow=ulaw
;deny=0.0.0.0/0
allow=gsm
;permit=10.99.19.89/255.255.255.0 ; replace with your network settings
;insecure=invite,port

[IMSI410060414559093]
callerid=sid <6004>
canreinvite=no
type=friend
host=dynamic
;nat=yes
;secret=pass ; put a strong, unique password here instead
context=users
allow=ulaw
;deny=0.0.0.0/0
allow=gsm
;permit=192.168.20.110/255.255.255.0 ; replace with your network settings
;insecure=invite,port

extensions.conf

[general]

autofallthrough=no
[users]
exten=>s,1,waitexten(5)
;exten=> s,1,Answer()
;exten=> t,1,Answer()
exten => 6001,1,Answer()
exten => 6001,2,Playback(demo-echotest)
exten => 6001,3,Echo()
exten => 6001,4,Playback(demo-echodone)
exten => 6001,5,Hangup()

exten=>6001,1,Dial(SIP/IMSI410035101450065,60)
exten=>6002,1,Dial(SIP/IMSI410018230446233,60)
exten => 6002,1,Answer()
exten => 6002,2,Playback(demo-echotest)
exten => 6002,3,Echo()
exten => 6002,4,Playback(demo-echodone)
exten => 6002,5,Hangup()
exten=>6003,1,Dial(SIP/IMSI410070380139067,60)
exten => 6003,1,Answer()
exten => 6003,2,Playback(demo-echotest) ; Let them know what is going on
exten => 6003,3,Echo() ; Do the echo test
exten => 6003,4,Playback(demo-echodone) ; Let them know it is over
exten => 6003,5,Hangup()
exten => 1000,1,Dial(IAX2/aqsa/1000)
exten => 701, 1, Dial (IAX2/ainee/701)

What does

sip show peers

from the Asterisk CLI say? You have those peers defined to register to you, so you can do that to see if they registered to you okay.