I would be carefuly before spending money on foot of this advice.
What Sky are saying is that you need a phone line, with a phone socket into which you can plug your Sky Box(es). You do not need a phone, as such, in the sense of a phone handset. In fact, be careful of certain VoIP DECT phones that do not provide any telephone socket at all (although you may not have any luck with any VoIP phones as described below).
Sky require you to have a phone connection for multiroom for use with Sky Interactive and Pay Per View. You can get out of this requirement for a non-multiroom contract by paying a fee, but for some reason they require you to stay hooked up for multiroom (probably to make sure that it's not used to divert the multiple Sky cards to other locations).
The phone line you use has to support dial-up to a modem, which is essentially what the Sky box does at regular intervals. This generally works fine over an old Eircom copper wire analog phone line to your house.
A VoIP phone line is one that connects over the internet, usually via a broadband connection. Often (but not always, hence my warning above) they provide an analog telephone adapter (ATA) that lets you plug in a regular analog telephone handset (or your Sky Box). HOWEVER, an ATA generally applies encoding/decoding including compression to the phone signal, which very often means that modem dial-up simply won't work.
I have no idea why UPC is telling you that you need a VoIP phone -- if you have a phone service from UPC, that's exactly what the phone socket in the back of your UPC router is -- an ATA connected to UPC's VoIP service. Some people have reported these working with Sky Boxes, many have not. But the same is going to be true of any VoIP setup -- you may have success and you may not, depending on vagaries of setup and VoIP provider. If you use a different ATA you will have to set up an account with a VoIP provider. If you're not very technically savvy and/or lucky, you may very well not get this working for your Sky Box. Also, as far as I know you need a phone line connection for each multiroom box.
If you really want to try it, the simplest setup is a Cisco ATA which you can buy online for about 50 euro. However, if I was you, I would just accept that Sky multiroom without an old analog phone line is impossible or unreliable. If your box fails to successfully dial up Sky once a month they may charge you penalties, basically amounting to an additional Sky subscription for each box. Some Sky support people have advised that Sky does not work with VoIP. If you are able to get reliable non-conflicting advice, good luck to you.
As a matter of interest, if you have UPC already, why not get UPC multiroom?
EDIT: As newirishman says above, your best bet is actually to just try it with the UPC phone connection and see if it works. But bear in mind you will need to string cables around so that each multiroom box can be plugged in, which may or may not be impossible.
Another thing that occurs to me is how stupid and self-defeating this is on Sky's part. Apart from losing them customers left right and centre, certain VoIP providers let you set your caller ID for phone calls, which means that you could easily take one of your multiroom boxes to your pub or holiday home in Spain and use it from there -- the very thing they are trying to prevent.