So I never knew this till today but I thought you needed CAT5 for VoIP but this business is running VoIP on CAT3.
Don't you need the extra wires for the voice, data and power?
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Ethernet specifications state four pairs, even if only two are used. The category rating system is a set of signal specifications, independent of the number of pairs. (there are 25, 100, etc. -- even 500 -- pair CAT3 trunk cables.)
As long as there are four pairs, a VoIP phone can be powered and connected over a CAT3 ethernet cable.
VoIP runs over IP, so it is independent of layer-1. The only sort of restriction based on layer-1 would be the bandwidth required. Voice is not separate from data on the wire.
You may be meaning a VoIP phone, which may be powered by PoE, but most have wall-warts, too. If the Category-3 cable has all four pairs, there is no real reason the it couldn't support PoE, too. Most copper PHY can support 10/100/1000 GB. If you have a PoE 10/100/100 port, you could run it at 10 Mb with PoE.
Allow me to clarify that It is possible to use cat3 cabling for IP phones in addition to POE.
Since the early 1990s, prior to many IP standards and written documentation for LAN Networks I have connected successfully many computers using only 2-pairs on a cat3 cable. Additionally, you can use 1-cat3 cable split out to 2-data jacks to run 2-computers. But, cat3 is limited to 16mbps, more commonly seen as 10-mbps. A POE enabled switch will provide power on pairs 2 and 3; pins 1,2,3 6. This is known as phantom power POE running on same pairs in which data is transmitted and received.
Let's discuss cat4 cable, very rare to see but supports up to 20mbps only and same configuration as above applies. A POE Adapter provides power in ununsed pairs 1 and 4; pins 4,5,7,8.
Then we come to Cat 5, 5e, 6, and 7 as discussed on the internet.