The Big VoIP Post

As self proclaimed VoIP-expert (and so does my boss say) I decided to create this blog post. English is not my normal language, already my apologies for the mistakes! Let me know if you find one 😉

The reason of this blog is partly because of my job. As already said, I’m a self proclaimed VoIP-expert and I’m lucky my boss also thinks like that. Because of this, I get all phone calls from customers who have problems with VoIP. Also I’m testing small PBX devices so I can give a fair shot of what the pro and cons are. I hope you enjoy the read. If you disagree, well.. let me know and even better, tell me why you disagree. Perhaps this will be updated!

VoIP in general

First of all, VoIP is really not that hard. The biggest difference between old school telephones and VoIP is basically the network. You probably think now, that’s pretty obvious and it is. Lot of customers don’t know this and there’s the first problem. While plain old telephones working with 2-wires, are the 2 wires replaces by UDP traffic over IP when using the standard VoIP. The customers telephone system has to convert that digital data into analog data which a phone will understand. The audio will be packed into small packets (UDP) and transported over the Internet. As you can imagine now, the packets had to come in the telephone system with a good stream and preferable directly behind each other. Back to the customer who doesn’t know that VoIP is traveling on network and thru routers. These people go to the local computer shop and buy the most cheap router there is. Result is a very bad VoIP connection (caused by this cheap router who cannot handle the traffic well) and the provider gets the blame. It’s the same as buying a huge and beautiful caravan while having a T-Ford pulling the thing. So, my first advice:
Use a decent router if you want to use VoIP!

In my tests, I’ll be using a Asterisk 1.4-based trunk which is directly connected to the Internet. It supports DDI and T.38 thru G711.alaw. Also support for G711.ulaw and G729, but these don’t work with T.38.

If you’re not aware what DDI and T.38 are, it’s probably better to read some wikipedia articles about this. 😉

Telephone systems / PBX

Hardware solutions using old telephone systems

The first systems I tested with were the default PBX systems which were originally older phone systems and with some added hardware they become suitable for VoIP. These systems are usually set up by old phone-expert with almost no knowledge of networking. Most common are Siemens, Panasonic, Alcatel and probably some more.

The problem is that these systems usually aren’t capable of firewalling. So, they put a cheap ass router before it and suddenly they have one-way audio, cracking sound and much other inexplainable problems. At one of the customers I found a Netgear firewall. Great systems, but terrible in VoIP transporting. Especially when a PBX uses 2 IP addresses to do VoIP (control IP and the UDP Stream IP). The Netgears SIP Helper cannot handle this. Panasonic PBX is known to do this. Ask your local PBX supplier how your system works and if it uses only one IP address, use the SIP helper. It will prevent problems!

All the above systems work good with a decent router in front of it. Put a nice Cisco there and it works. DDI has to be on for all named systems, else they simply refuse to work. T.38 is also working like a charm.

For old hardware VoIP/PBX solutions (Panasonic, Siemens, Alcatel):

  • It works when good configured
  • T.38 Support and DDI Support are perfect.


  • Pretty expensive
  • No sense of security, so external firewall is needed
  • Trunk providers has to pay to get support from the maker.

Software Solutions

There are a plenty of software solutions. I can’t test them all because there are simply too much. I like the software solution the best because the whole networking/firewalling part could be done by the operating system. Some of them are even free (like Asterisk) and really work like a charm in basically anything. Also some of them (Swyxware) have huge problems with T.38 (Swyx only supports G771ulaw for T.38 which doesn’t work everywhere). Some software tools cannot use DDI (XLite) and depending on which software you choose, it will only support one line (again X-lite).


  • Networking/security done by operating system
  • Support of T.38/DDI/Codecs depend on software
  • Can be free.


  • Can be relatively expensive.
  • Not always decent support.
  • Not all software

Home office all-in-1 devices

In this category I only have 2 tested. Both devices are basically doing the same.

– FritzBox 7120 –

The all known Fritzbox. Beautiful device which does routing, firewalling, ADSL / Dialup and being a (wireless) switch. Also T.38 fax works perfectly and with the good working Fax2Email functionality it will save a few trees as well. There is only one major Con on this device, there is and there never will be DDI support. So, you can not make it a real PBX in an office environment (unless all phones can ring at the same time).

– Draytek IPPBX 2820n –

This can do exactly the same as the FritzBox with 2 differences. DDI is working perfectly (even with menu’s, call forwarding based on phone number and/or office hours). So, I thought I found a winner. With the latest firmware release they say to support T.38 and that’s the other difference. T.38 doesn’t work. Again, a big con. Voicemail to Email works nice and even fallback to PSTN works. Really a device which can make it in the VoIP world as cheap device

– Gigaset 800 series –

Gigaset is a bit odd one here. It’s not an all-in-one device but only a PBX. In the background it runs Linux and Asterisk. Gigaset only created a nice interface with Asterisk and ask a lot of money for it. Because it runs on Asterisk, everything works. So, nothing more to add except I think it’s too expensive.


Only a few tested and these simply work. T.38 support is questionable. So far, only a Linksys is tested and that one worked.

VoIP Phones

Well, generally these work with DDI and not with T.38. One exception, the Gigaset. This one is not supporting DDI at the moment.

What I hope to reach with this blog

Well, I hope that some vendors will make their VoIP products better. I also hope that all people who are thinking of starting to use VoIP, also think about how the VoIP device (or devices) will be connected to the network. Use a decent router. The router is 80% responsible of how you’ll experience the quality of VoIP! Remember that!

If you have any questions, feel free to respond below. I’d love to see comments and hopefully I made some things clear.

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6 Responses to The Big VoIP Post

  1. He Bart, leuk stuk om te lezen! Wij werken momenteel met 3CX. Zou je eens willen brainstormen over hoe we dit kwalitatief zo hoog mogelijk kunnen gebruiken? Qua router configuratie (Juniper Netsreen)o.a.

    • whizzi says:

      Hoi Hein!

      Leuk dat je de site gevonden hebt ;). Wat betreft 3CX, daar heb ik relatief weinig ervaring mee. Wat ik wel weet is dat elke zelfrespecterende router geschikt kan zijn voor VoIP, mits de juiste configuratie wordt gedaan. Denk hierbij aan NAT, eventuele SIP Helpers en zelfs QoS kan hierin positief werken. Bij VoIP-centrales waar de “control channel” en het “data channel” andere IP adressen hebben, moet de SIP Helper zelfs uit staan (Panasonic o.a.). Als laatste is vooral belangrijk wat een router te doen heeft. Doet een router alleen VoIP verkeer, dan heb je weinig tot geen QoS nodig. Bij een router waar ook veel “internet” verkeer over gaat, is QoS wel aan te raden en daarbij een goede SIP Helper (of een hele goede NAT/NAT-tabel die niet vol loopt).

      Uiteraard kun je een keer bij de bekende leverancier een test account aanvragen en dan met de favoriete helpdesker hierover brainstormen 😉

  2. Mody says:

    Dank je wel voor de uitgebreide toelichting. Vraag!

    Ik gebruik thuis ook viop om te bellen al 4 jaar zonder probleem en ik maak gebruik van de router van de internet provider (Online) en heb ik siemens sip telefoon. Echter sinds een paar dagen terug opeens de telefoon gaat maar als ik opneem dan hoor ik niets terwijl de beller denk dat ik niet opgenomen heb, maar ik kan well bellen zonder probleem maar niet gebeld worden. Graag heb je een tip voor mij of een oplossing.

    Alvast bedankt

    Met vriendelijke groeten


  3. Jan says:

    Hoi Bart,

    Het is op zich plezierig te lezen dat de Draytek 2820N het prima zou moeten doen. Helaas is dat bij ons niet het geval. Wij hadden via, via de aanwijziging gekregen om de toesetllen van Gigaset metd e Draytek 2820N te combineren. Na 3 dagen configurenen door een ICT-VOIP bedrijf werkte het nog steeds niet. We hebben GigaSet en Draytek (importeur) erbij gehaald en alles laten testen. Met als resultaat : Draytek PBX 2820N vervangen door router 2920 i.c.m. een Gigaset T300. De toestellen zijn de Gigaset DE700/410 Pro.

    Waar we nu toch tegen aanlopen is dat we door verschillende klanten worden benaderd dat de telefoonlijn “dood stil” is als ze opns bellen. of te wel er is geen contact. Uunet algebeld, maar dat ligt niet aan hun SIP-trunk!?

    Wat het wel is weten we niet, maar lastig is het wel.

    Heb jij een idee….?

    Alvast bedankt,



    • whizzi says:

      Beste Jan,

      Als ik het goed begrijp zit je VoIP-dienst bij UUNet. Hun kunnen een trace uitvoeren om te controleren of de datastroom met geluid wel op een goede manier wordt afgeleverd. Als dit het geval is, dan zal toch echt weer op de router moeten worden gecontroleerd of deze SIP-ALG heeft aan staan (of een SIP-helper).

      Met een dergelijke trace is ook uit te zoeken waarom de Draytek 2820N niet werkt, deze heeft wel enkele limitaties, maar heb ik tot nu toe probleemloos weten te koppelen met willekeurige centrales.

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