Hi Leanne,
This is not likely the best solution without using IP Phones as part of the solution :( Most PBX's use some sort of proprietary signalling that won't work with the UC520. Even if the phones were analog the costs for FXS ports to get them working would be close to the costs of aquiring new phones.
To fully leverage the overall SMB solutions offered by the UC520 the customer should use Cisco IP Phones. The upside is that there are many Models available that vary considerably in price so not everyone needs the most expensive set.
Hope this helps!
Rob
