Powerline is essentially a point to point DSL connection that uses the existing A/C power infrastructure as a way to transport Ethernet traffic... The phone itself if just another IP endpoint like any PC, printer, server, etc., so it should work provided there is enough bandwidth and other network parameters like latency and delay are acceptable.
That being said, in actual practice it will depend on the infrastructure in the building and the amount of traffic being carried, if this is for one or two phones and PCs it generally shouldn't be a problem. I had a customer try this when they took over an office area on the opposite end of building that they took over some office space and put 6 phones, 5 PCs, and printer in to a Cisco switch and then provided a feed to the switch via Powerline from the main switch... lets just say in day or two we were down there running cable.
