sip.conf

[serverA]
type = peer
host = 192.168.1.101
username = serverB
secret = apples
context = incoming
disallow = all
allow = alaw

extensions.conf

exten => _5XXX,1,Dial(SIP/${EXTEN}@serverA)

sip.conf

[serverB]
type = peer
host = 192.168.1.102
username = serverA
secret = apples
context = incoming
disallow = all
allow = alaw

extensions.conf

exten => _6XXX,1,Dial(SIP/${EXTEN}@serverB)

sip.conf

[general]
...
register => username:password@your.provider.tld
...

[myprovider]
type = peer
host = your.provider.tld
username = username
secret = password
; Most providers won't authenticate when they send calls to you,
; so you need this line to just accept their calls.
insecure = invite
dtmfmode = rfc2833
disallow = all
allow = alaw

extensions.conf

exten => _XXXXXXXXX,1,Dial(SIP/${EXTEN}@myprovider)

str.150