The simple approach

SIP

[general]
port = 5060                 ; port to bind for sip connections
bindaddr = 0.0.0.0          ; ip to bind for sip connections
context = from-sip          ; default context for incoming sip calls
;
disallow = all              ; disallow all codecs, we want to enable, 
allow    = gsm              ; what we deem is necessary
allow    = ilbc
allow    = speex
allow    = g729             ; g729 only works for pass-thru, if you haven't bought a license
allow    = g726
allow    = ulaw             ; i would normally not allow ulaw, because it's high bandwidth,
                            ; but if you want to use free world dialup services, it's the
                            ; only codec they support
;
register => 12345:not_my_password@fwd.pulver.com/marlow-fwd                    ; these make it possible to get calls
register => 17471234567:not_my_password_either@proxy01.sipphone.com/marlow-sip ; in from these services.
;
;
; Dial out to fwdnet
;
[fwd]
  type=peer
  secret=not_my_password
  host=fwd.pulver.com
;
;
; Dial out to sipphone.com
;
[sipphone]
  type=peer
  secret=not_my_password_either
  host=proxy01.sipphone.com
;
;
; This is a local extension running kphone, firefly or likewise
;
[marlow]
  callerid=("Martin List-Petersen" <3986>)
  type=friend
  secret=not_my_password_anyway
  host=dynamic
  context=intern
; canreinvite=no ; "canreinvite=no" is needed, so that all calls allways go through asterisk. This is
;                  needed, when you have the SIP client on the LAN and want to use asterisk as a proxy.
;
; qualify=200
; nat=yes        ; these two are needed, if you asterisk box sits on a public ip
;                  and your sip client/ata box/ip phone behind a nat firewall
;                  usually you would not need a stun server then.
    

Extensions

[general]
  static=yes
  writeprotect=no

[globals]
  ;
  ; The name to use on callerid
  ;
  MARLOW_CID=Martin List-Petersen

  ; FWD ID for callerid
  ;
  MARLOW_FWD=12345

  ; sipphone ID for callerid
  ;
  MARLOW_SIPPHONE=17471234567

[internal]
  ;
  ; local extensions
  ;
  exten => 3986,1,Dial(SIP/marlow,60)           ; call SIP extension "marlow" for 60 seconds,
                                                ; if extension 3986 is called
	exten => 3986,2,Voicemail(u3986)              ; if we can't connect to "marlow" or after seconds 
                                                ; go to the unavail VM
	exten => 3986,102,Voicemail(b3986)            ; if busy, go to the busy VM

  ;
  ; Voicemail System
  ;
  exten => 9999,1,VoiceMailMain(${CALLERIDNUM}) ; extension 9999 is the VM system,
                                                ; go directly to callers VM
  exten => 9999,2,Hangup

[from-sip]
  ;
  ; default extension for calls from SIP
  ;
  exten => s,1,Dial(Local/3986@internal/n)

  ;
  ; calls from Free World Dialup (FWD)
  ;
	exten => marlow-fwd,1,SetCIDName(FWD - ${CALLERIDNAME}) ; indicate that the call came through FWD
  exten => marlow-fwd,2,Dial(Local/3986@internal/n)

  ;
  ; calls from sipphone
  ;
	exten => marlow-sip,1,SetCIDName(SIP - ${CALLERIDNAME}) ; indicate that the call came through sipphone
  exten => marlow-sip,2,Dial(Local/3986@internal/n)

[outbound-internal]
  ;
  ; include local extensions
  ; 
  include => internal

  ;
  ; include SIP accounts
  ;
  include => fwdnet
  include => sipphone

  ;
  ; include tollfree calls
  ;
  include => tollfree

[default]
  ;
  ; include from-sip for default. We don't use it, but it might be a good idea
  ;
  include => from-sip

[fwdnet]
  ;
  ; fwdnet extensions
  ;
  exten => _1393.,1,SetCallerID(${MARLOW_CID} <${MARLOW_FWD}>)   ; set my callerid and name for FWD
  exten => _1393.,2,Dial,SIP/${EXTEN:4}@fwd                      ; dial the number i wish to dial on FWD
  exten => _1393.,3,Playback(invalid)                            ; this did not work out
  exten => _1393.,4,Hangup
  exten => _1393.,103,Busy                                       ; the destination was busy
                                                                 ; dial jumps to +101 if busy

[sipphone]
  ;
  ; sipphone extension
  ;
  exten => _1747.,1,SetCallerID(${MARLOW_CID} <${MARLOW_SIPPHONE}>)
  exten => _1747.,2,Dial,SIP/${EXTEN:4}@sipphone
  exten => _1747.,3,Playback(invalid)
  exten => _1747.,4,Hangup
  exten => _1747.,103,Busy
  
[tollfree]
  ;
  ; terminate toll-free no.'s via fwdnet
  ;

  ;
  ; US toll free access
  ;
  ; +1-800
  exten => _1800.,1,SetCallerID(<${MARLOW_FWD}>)
  exten => _1800.,2,Dial,SIP/*${EXTEN}@fwd
  exten => _1800.,3,Playback(invalid)
  exten => _1800.,4,Hangup
  exten => _1800.,103,Busy

  ; +1-866
  exten => _1866.,1,SetCallerID(<${MARLOW_FWD}>)
  exten => _1866.,2,Dial,SIP/*${EXTEN}@fwd
  exten => _1866.,3,Playback(invalid)
  exten => _1866.,4,Hangup
  exten => _1866.,103,Busy

  ; +1-877
  exten => _1877.,1,SetCallerID(<${MARLOW_FWD}>)
  exten => _1877.,2,Dial,SIP/*${EXTEN}@fwd
  exten => _1877.,3,Playback(invalid)
  exten => _1877.,4,Hangup
  exten => _1877.,103,Busy

  ; +1-888
  exten => _1888.,1,SetCallerID(<${MARLOW_FWD}>)
  exten => _1888.,2,Dial,SIP/*${EXTEN}@fwd
  exten => _1888.,3,Playback(invalid)
  exten => _1888.,4,Hangup
  exten => _1888.,103,Busy

  ;
  ; Netherlands toll free access
  ;
  exten => _31800.,1,SetCallerID(<${MARLOW_FWD}>)
  exten => _31800.,2,Dial,SIP/*${EXTEN}@fwd
  exten => _31800.,3,Playback(invalid)
  exten => _31800.,4,Hangup
  exten => _31800.,103,Busy

  ;
  ; France toll free access
  ;
  exten => _33800.,1,SetCallerID(<${MARLOW_FWD}>)
  exten => _33800.,2,Dial,SIP/*${EXTEN}@fwd
  exten => _33800.,3,Playback(invalid)
  exten => _33800.,4,Hangup
  exten => _33800.,103,Busy

  ;
  ; UK toll free access
  ;
  ; +44 500
  exten => _44500.,1,SetCallerID(<${MARLOW_FWD}>)
  exten => _44500.,2,Dial,SIP/*${EXTEN}@fwd
  exten => _44500.,3,Playback(invalid)
  exten => _44500.,4,Hangup
  exten => _44500.,103,Busy

  ; +44 800
  exten => _44800.,1,SetCallerID(<${MARLOW_FWD}>)
  exten => _44800.,2,Dial,SIP/*${EXTEN}@fwd
  exten => _44800.,3,Playback(invalid)
  exten => _44800.,4,Hangup
  exten => _44800.,103,Busy

  ; +44 808
  exten => _44808.,1,SetCallerID(<${MARLOW_FWD}>)
  exten => _44808.,2,Dial,SIP/*${EXTEN}@fwd
  exten => _44808.,3,Playback(invalid)
  exten => _44808.,4,Hangup
  exten => _44808.,103,Busy
    

Starting it up