Re: Voice quality
VoIP sounds shite only if you don't bother to QoS.
A VoIP channel is TINY in terms of bits-per-second. Sure latency will delay things. Jitter will make it wobble. But those don't tend to matter for a... what? 64Kbps stream over a 10Mbps channel? That's a HUGE amount of loss before you're affected. Latencies of 100ms on voice traffic are not noticeable and if you have more than 100ms ping to even the other side of the planet, you have bigger problems on your connection.
But what matters is that your phone call doesn't get superseded by Fred over in the office loading his Facebook. His packets can wait, and he'll never notice. Yours can't.
It's very easy to demonstrate. When a workplace first gets a VoIP phone in-house, warn them that it will happen. You can put in the most expensive switches in the world and massive redundant fibre connections and all sorts, and even on switchboard-only calls, it'll work. For one phone. Two phones. Three phones. But before you even get into silly numbers (say a dozen), or if there's a particular time when everyone logs in... the phones will start to distort and cut out. It doesn't matter how much money you throw at it at that point.
At that point, everyone starts getting rubbish service and it sounds bad. So you VLAN off the voice traffic, and apply QoS to it. Now watch as you can go to 100, 200, 300 handsets and no problems. On the same networking, the same expensive (or cheap!) switch. The same Internet line.
The problem with telesales people on VoIP is either that they don't have the IT people to know this, or they are all working from home over their home ADSL router (more common than you might think... lots of people do telesales jobs from home, dial into the switchboard from a phone they are given to plug into their router, and off they go). Nobody sets QoS properly. And it affects NOTHING else when you do it right, while making the phones all "just work". I've demonstrated this phenomenon on everything from the cheapest Netgear to the most expensive Cisco, with less than a dozen phones each time. Works fine at first, then ordinary network usage interrupts it and it fails. Apply QoS on the same switch and then you can expand enormously without issue.
I've seen contractors who carry around Ethernet IP phones and just plug them into people's networks expecting it all to magically work... and invariably they say it works where the IT is well-managed and doesn't anywhere else they try it - even if they're the only person on the network. Because the QoS isn't automatic.
And QoS applies not just to the local network, but to your wireless ("Airtime Fair Sharing") and outgoing packets too. Your router has to know to deal with the voice packets FIRST before it worries about your Counterstrike game. It has to respect QoS and pass it on (it'll likely be ignored by the ISP, but you never know) just the same as the switches. Your Whatsapp traffic isn't QoS'd because I think it goes out over untagged encrypted protocol which your smartphone / wireless / router doesn't understand or respect. That's why it does that.
The number of times I've had calls FROM people selling me VoIP where I literally can't hear their call (and it's not us... at various places and times I've had analog, ISDN and SIP so we know our calls were clear).
When we started buying local IP handsets, the problem came within a dozen and I QoS'd and four years and a hundred handsets later we're fine. When we started going down the line of SIP trunking, I did the same - made sure the traffic was VLANned, that entire VLAN was QoS'd on the switches, prioritised on the router, firewall and wireless points. Made sure that the outgoing SIP ports were forced to max priority so they retained that QoS when they went out to our ISP, etc. Literally never had a problem, even with user's maxing out the connection on an hourly basis.
Voice traffic doesn't care about bandwidth and retransmission, like other technologies. TCP will just "try again" so fast that you'll never notice a problem. But VoIP needs to be jumping the queue for every tiny little packet it sends because it NEVER tries to send it again, it's already too late by then. If it can't jump the queue - from the phone to the network to the switch to the firewall to the router to the Internet - then it will be bad. If it can jump the queue, it's literally so miniscule that nothing else will notice or care. The actual bandwidth it consumes is pathetic.
To be honest, even "wireless" IP phones have more problems than cabled ones. Because you can't stop someone on the same channel but another SSID or just plain interference from "jumping the queue" and holding up the voice traffic.
If you are expecting VoIP and you're not in control of QoS... you're on your own. It might work, it might not. If you are MANAGING VoIP - apply QoS from day one on everything in the path. Then, quite literally, you can run a entire company switchboard from a dodgy old ADSL line.
As an aside, we abandoned all our analog and ISDN lines last year, after many years of waiting for approval to do so. They were more likely to provide poor performance (everything from rain affecting the cables, to things literally falling off the telegraph poles) and we had more faults on ISDN than I care to remember. We retain one emergency line only so we can dial 999 if the system goes off. But everything else is entirely SIP. I haven't had a complaint about call quality for a year.