Introduction.
This post is going to briefly cover how to use asterisk to be a proxy between an old ISDN System and a SIP Trunk. This example also shows how to modify the dialed number to add in the area code you are dialing, for example if your users are used to just dialling 6 digits to get to a local business etc.
Example Explained.
In our example we have an ISDN PBX and we want to integrate a SIP trunks, so that we can take advantage of lower calling costs, IE you might have a SIP trunk from a USA provider and one from the UK, you would send all USA numbers out to the USA trunk and they would forward the call at the cost of a Local call, while still being able to dial UK numbers via the UK trunk etc etc…
This is called LCR or Least Cost Routing, and could save you some money on your telephone bill.
Right back to our example, we have an ISDN PBX, a ISDN to SIP convertor, and an asterisk server to perform both LCR and a simple dial plan.
Inbound:
SIP Provider > Asterisk > ISDN Gateway > PBX
Outbound:
PBX > ISDN Gateway > Asterisk > SIP Provider
In the example all we are doing is taking the call from the ISDN gateway checking to see if we need to add the local area code if calling locally and forward the call on to the SIP provider.
We could have a few SIP providers and get very granular on how we want the call to be routed and to where.
Example Config.
sip.conf
; Provider SIP Trunk
[Trunk-Provider]
type=friend
context=from-provider
deny=0.0.0.0/0.0.0.0
permit=IP-ADDRESS
host=IP-ADDRESS
username=USERNAME
secret=PASSWORD
language=en_GB
disallow=all
allow=alaw
dtmfmode=rfc2833
; ISDN SIP Trunk
[ISND-Trunk]
type=friend
context=from-isdn
deny=0.0.0.0/0.0.0.0
permit=IP-ADDRESS
host=IP-ADDRESS
username=USERNAME
secret=PASSWORD
language=en_GB
disallow=all
allow=alaw
dtmfmode=rfc2833
extensions.conf
[from-isdn]
; from isdn with area code
exten => _X.,1,Dial(SIP/${EXTEN}@Trunk-Provider,1000)
exten => _X.,2,hangup
; from isdn, no area code, six digit, prepending area code
exten => _NXX.,1,Dial(SIP/+0115${EXTEN}@Trunk-Provider,1000)
exten => _NXX.,2,hangup
[from-provider]
; from SIP Provider, send to ISDN
exten => _X.,1,Dial(SIP/${EXTEN}@ISND-Trunk,1000)
exten => _X.,2,hangup
lll