Wired vs Wifi VoIP calling
VoIP Call Quality
The Internet was designed for scalability. It gracefully handles the addition of new devices to a network in such a way that there's hardly any performance degradation by adding an extra node. It achieves this using a technology called "packet switching" which allows the same channel to be used by as many connections as necessary. This is why you're able to use multiple Internet applications at the same time on the same device. A telephone on the other hand uses "circuit switching" technology which reserves the line for communication between the two callers. This is why phones have traditionally not handled multiple connections well. In the old days, you could often get a message saying that the lines were clogged up.
This was a great solution because dedicating a channel to just one session results in a massive waste of bandwidth since it's not used to its full capacity and is one of the reasons for the success of the Internet. However, it's undeniable that packet switching is ever so slightly more "sketchy" than circuit switching. Most of the time this is not noticeable at all. The TCP protocol is very resilient and recovers well from packet losses. But for applications such as VoIP, even small degradations in call quality are noticeable. This is why it's important to implement strict QoS restrictions on VoIP traffic.
Wifi VoIP Calling
Quality of Service (QoS) measurements are very important for VoIP and this is true for both wireless and wired services. Good routers these days automatically prioritze incoming VoIP traffic to ensure that end users get the best possible call quality. Jitter, packet loss and latency are three factors that can destroy a VoIP call and make it almost unusable.
All things being equal, wifi is just a tad more unreliable than wired connections. For almost every application ever written this is a non issue. You won't notice any difference in your videos loading or the speed with which your webpages render. But VoIP users can make out the variation. Small clicks and slight jitter can be heard sometimes. Wifi is affected by factors such as nearby electronic devices, walls and other obstructions.
The moral of the story is that wired connections are absolutely ideal with proper QoS rules in place. Wifi is an excellent alternative that is far superior to the standard wireless Internet provided by the telcos. It's just useful to keep in mind which is the best medium all things being equal. Businesses will almost always have wired HD voice IP phone systems in place linking to their SIP VoIP proxy servers for maximum reliability. So this shouldn't be too much of a problem for them.