While wireless is a good, convenient, possibly necessary thing for a company to have and use, you don't own the airwaves, and you must be willing to accept any interference that disrupts your Wi-Fi, even if it disrupts your business. Having a wired infrastructure can save your business a lot of money. I don't think you really want to bet the company on something that is out of your control. Also, a proper wireless infrastructure (WLCs, WAPs, and supporting hardware) can be as much or more than a switched network infrastructure, and it takes work to keep the wireless network properly tuned.
The problem with VoIP over Wi-Fi is that QoS, necessary for VoIP, is problematic for Wi-Fi. Wired VoIP phones are actually switches, too, so you can connect a PC through a phone, using a single switch interface for both devices. That saves on the number of switch interfaces necessary.
The company where I work has people drooling over the idea of wireless-only sites, but it just has not proved practical.
The ANSI/TIA/EIA 568, Commercial Building Telecommunications Cabling Standard actually calls for a minimum of two telecommunications outlets per work area:
Each individual work area shall be serviced with a minimum of two
telecommunications outlets. One will be associated with voice and the
Edit for the comments:
If everyone suddenly switches to Wi-Fi for primary network access, you must be prepared for disappointment. The users often have expectations from using Wi-Fi at home that you simply cannot meet in the workplace. For instance, a home user have very little wireless contention on the half-duplex medium, but there will be much more contention in the office. Only one device on a particular Wi-Fi channel can use the medium at any given time, and the Wi-Fi protocol enforces sharing.
A home user downloading files doesn't have much competition for the WAP, but given multiple users on a WAP trying to simultaneously send and receive files, you will notice it, and network drive shares can seem very sluggish. On-line meeting software, like Lync or Skype, can be greatly affected, too.
It's not all bad. Things like limited networking for things like meeting rooms get much easier to remedy. The key is to manage user expectations, but don't expect that you can actually do a good job of that when so many people have such high expectations based on what they experience at home every day. There are companies that have made the transition and enjoy it, but how they use the network may not even resemble how your company uses the network.
You need to be prepared for some users to simply revert to wired networking for most things. The more users that do this, the better the wireless network performs for the remaining wireless users. A natural balance will be struck, but it may take some time as users switch back and forth before settling to what they prefer.
Also, you really need to hire a company to do a wireless site survey before you decide on how to design the wireless network. This will give you the WAP quantities, WAP placement, frequencies to use on each WAP, power levels for each WAP, etc. Shortly after you implement the wireless network and start using it, you should perform a follow-up wireless site survey. This will help you fine-tune things for optimal performance. It is not a bad idea to do this every year or if you notice problems. Things one the airwaves do change. For example, you could have a neighboring company move in and crank up their Wi-Fi radios to full power on a frequency that you are using close to their location.
All in all, between the expanded number of WAPs, the switches required for the WLCs, and the users that prefer to remain on the wired network, you will probably not save many switch ports, if any. You may find that you use more switch ports (switches do come with fixed port quantities).