I guess it depends what you mean by VoIP. If all you want to do is make calls via internet many of the newer cell phones have the ability to make skype or other "VoIP" calls directly over a wifi network. There are of course many phones that will directly talk to skype or vontage or whatever via wifi also.
Now if you mean you are using something like a H.323 or SIP based phone to connect to a corporate VoIP network then it gets more complex but there are phones that will connect directly via wireless it just gets complex if you have a vpn layer on top of it. Lately I have been using a app that runs on a android based cell phone that lets me use SIP over WIFI to access a corporate network.
I'm part of a Tech Support department of 50 located along the Gulf Coast. In years past we have found hurricanes and other disasters may have forced Engineers to work from home or out of hotel rooms for several weeks. Since that time we are now using VoIP phones that require a power over Ethernet (PoE) LAN connection. Along with the phone we already have an external power brick that we can take home that supplies the PoE LAN connection. However, that requires that you have access to the router so you can run a physical cable from the router to the power brick.
That's fine when you are at your own house and have access to your own router. But in the case of a hotel, you can't run a cable to the router. Many hotels have gone to wireless only and don't have a physical LAN jack to plug into. So I've been looking at something like the Netgear WNCE3001 Dual-Band Internet Adapter. That will supply a physical LAN connection from WiFi only. But it does not have PoE. So now if we are going to work from a hotel room, we need the VoIp phone, the power brick and the WiFi Internet adapter. 3 seperate items to lug around.
Is there such a device that would allow you to pickup a WiFi signal in a hotel room and has a physical LAN jack that is also capable of PoE? I could at least lose the power brick then.