Grandstream HT 503, PBX, SIP: my recipe for VoIP
As some of you may know, I live about 2,037 km far from my family in Italy and since I moved to Oslo I always had some problems communicating with some people in my family.
The main reasons are that they are not computer literate (like my mom), they don't have a (decent) internet connection (like my sister) or simply they are not that often online (again like my sister).
So, what did I do to solve the problem?
Scenario
At my parents' home in Italy there is an ADSL flat internet connection, and a PBX which currently serves 3 standard telephones around the house.
At home here in Oslo I have a cable flat internet connection provided by Get and no telephone lines.
The solution
My idea was simple, use an interface between the PBX and the internet, so that I could use my pc and VoIP to call it and interact with the PBX.
This interface is commonly known as ATA and it's easy to find such devices for home use at a reasonable price.
The advantage of having a PBX is clear; the ATA just becomes another telephone connected to the PBX and can behave exactly like them.
In other words it can call the other phones and receive calls from them, plus can dial external numbers.
Grandstream HT 503
I spent quite some time researching, annoying my buddies on the Perl.it IRC channel and reading manuals to understand what I really needed and if a particular ATA was suitable for what I wanted to do.
All the phone terminology was new to me, but soon I understood that I needed an ATA with an FXO port for connecting it to the PBX.
The Grandstream HT 503 seemed to be the right device for me, with one FXS and one FXO port with separate SIP accounts, and so I bought one and spent quite a lot of time during my one-week Easter vacation in Italy trying to make it work... and I succeeded!
The configuration
I'm not going through all the details, but basically the whole system configuration required work both on the PBX, on the ATA and a stupid setting in the router.
If you want to know more feel free to contact me.
Router
This was actually the reason why at first I wasn't able to get the ATA work.
Since the SIP protocol uses several UDP/TCP ports for authenticate and transmit the audio packets, the router must have UPnP enabled.
ATA
I disabled the FXS port, since I don't have attached any analogic telephone to it, I only use the FXO, configured with a SIP account and the following relevant settings:
- PSTN Ring Thru FXS: no, which avoid calls to the ATA pass thru the FXS port before being forwarded to the FXO
- Number Of Rings: 1, which minimizes the time needed for an incoming call to be bridged to VoIP
- Unconditional Call Forward to VoIP, which automatically forwards every incoming call to the ATA to the SIP account I've configured in my softphone
- PIN for PSTN calls, which should avoid unauthorized uses of the PBX and the phone line
- DTMF via RFC2833, which allows me to dial password and numbers using tones with the keypad of my softphone
- Register expiration: 30 minutes, I lowered the threshold after experiencing some problems of ATA offline
PBX
On the PBX I changed the settings for the ATA internal number so that it does not ring when an external call comes in.
Normally all the internal telephones ring, but remember that I configured the automatic forward to VoIP and I don't want my SIP account to ring every time someone calls my parents.
Softphone
I currently use Gizmo, a free multi platform SIP phone software.
The only setting here was for the DTMF tones to use out-of-band RFC 2833.
In other words
What I can do now is:
1) call an internal phone at my parents' house:
From my softphone I call the SIP number of the ATA, when it picks up I hear a tone and I dial the password, if correct I hear the PBX tone and I can dial the number associated to any internal phone and talk with my parents :)
2) call an external number:
After calling the SIP number of the ATA and accessing the password protected PBX line, I can access the external line, then dial any number and let my parents pay :P
3) receive a call:
My mum can pick up any telephone at home and dial the internal number associated to the ATA, which after 1 ring forwards the call to my SIP account and if I'm connected I receive the call through the softphone.
4) receive a call from outside my parents' house:
Someone calls my parents' phone number and talk with them, then at some point my parents can forward the call to me simply dialing the internal number associated to the ATA and I can also forward the call back to any telephone at home,
But...
You may ask: why didn't you just pay some service like SkypeOut and call every number you want in Italy?
The answer is: well, because of several reasons.
First of all, why should I pay for calling my mother when I can do it for free (free as in we both pay already for internet connection)?
Second, Skype uses a closed proprietary protocol, which for people like me doesn't sound all that right.
Third (optional), because that way is too simple, where is the fun? ;)
Happy VoIPing! :)
Enclosure:
