When I was a contractor building IT architectures, it was common to find that people had a rather old phone system and were looking to move to something newer. The question they asked was always: what shall we do to get something stable but future-proof?
Phone lines or IP trunks?
Even in 2015 you seldom see an organisation that uses just IP trunks to call external numbers. ISDN lines are still the primary way to connect to the world, for the simple reason that they work and you can be absolutely sure – unless there is some kind of hideous hardware problem on the network – that the signal will get from end to end intact.
Most SIP (Session Initiation Protocol) services rely on people connecting to them over the internet, which means performance can be variable. If you are calling in your own country, however, it is often acceptable unless you are, say, a call centre that really cares about sound quality.
Happily, the big name telcos are starting to sell SIP services over their own local loop, providing a happy balance of decent quality on average for a price somewhere between ISDN and cheap SIP. I am surprised it is taking this long to become popular, but it will come over the next year or two.
Can I use SIP for cheap international calls?
Remember that the cheaper the provider, the worse the quality. The cheap providers generally hop between international backhaul providers so your service can vary between fairly rubbish and outright shocking.
Many of these international “service providers” are merely call forwarders making a fraction of a penny per minute on the calls they carry. If they can scrape another 0.1p per minute by switching to another overseas provider that just about keeps the call audible, they will do so.
Moving to SIP for international calls is for home users with relatives overseas; it is not for businesses.
Proprietary or SIP handsets?
Neither. Or both. These days the choice is not necessary. If you decide to buy a commercial phone system you will generally find that the vendor's handsets support both its own protocols and the SIP standard.
SIP support means vendors can at least flog you their handsets even if you decide to go for an open-source phone system such as Asterisk instead of their own model.
Interestingly, even the open-source mob have decided to go the other way: Asterisk now supports Cisco's own SCCP as well as the standard SIP protocol.
How do I connect the handsets?
One of the benefits of IP Voice is that you no longer need a direct connection between the phone and the PBX (private branch exchange). Most IP phones allow you to daisy-chain through them to connect the desktop PC, which means you need only one RJ-45 outlet per desk, not two.
If your phones can only do 100Mbps Ethernet and your users whinge that this isn't fast enough, tell them to get lost. Generally it is fine unless they are doing stuff like throwing high-res graphics and video around. If your phones can do Gigabit, you will have no problem.
Incidentally, I have had many clients and users moan over the years that the phone has been slowing down the PC. They were all wrong so don't fall for it.
Do I need QoS?
If you buy an IP phone system the installer will probably tell you that you need to implement quality-of-service (QoS) guarantees on your network and will offer to flog you some LAN switches that support it. You almost certainly don't need it if you have a decent (Gigabit Ethernet) LAN. With so much bandwidth going spare there will seldom be congestion.
The only time you want to care about QoS is if you are doing IP Voice over a WAN of some sort – and if you can afford a private WAN, you probably have decent QoS-capable equipment anyway.
A word of warning: if you want the provider to give you QoS over the WAN there is probably a fee.
How do I connect my sites together?
If money is tight, you can consider hanging your sites' phone systems together using SIP over the internet. You are, after all, calling yourselves so any dropouts will not be upsetting customers.
If you have sites located near each other, a traditional leased line is not a bad idea. These can cost next to nothing, particularly if you are in a big city, and you can use them either as an IP link or a telco-standard DASS II connection.
Don't forget, of course, that there is nothing wrong with doing SIP between the sites where it seems to work and ISDN elsewhere – or even having ISDN alongside SIP as a fallback if the internet decides to misbehave.
What about my fax machines?
Many commercial phone systems have at least a couple of analogue ports for fax. If the system you use doesn't, there are loads of IP/analogue gateways on the market that will give you the connectivity you need.
Most IP phone systems have a list of “supported” gateways, which means “gateways we have actually seen it work OK with”, so go for one of those.
Do I have to use SIP?
No. In fact, don’t unless you have to. If you are hooking two similar phone systems together and they have a proprietary interconnection protocol, use that: it is probably better optimised than SIP for that installation. SIP is very much a lowest common denominator.
Do mobiles do IP Voice?
Yes, but normally only if you install a SIP-based telephone application on them. Remember, your mobile is designed to use the cellular network and its IP stack uses that network as its transport.
Modern smartphone apps are getting cleverer and will try to do fun stuff such as switching your calls between the mobile network and the SIP/IP transport, but you will be using an app on your phone, not just the native phone operating software.
Can I build a free phone system?
Yes, absolutely. Asterisk is the one of choice: it is scalable and it works brilliantly (though it is not the easiest to configure unless you decide to spend a few minutes fighting to get the GUI to work for you).
Even better, Asterisk is supported by Digium, which is a vendor of telco (ISDN and analogue) cards for PCs, so even if you need ISDN connectivity you can still use it. ®