I have 550's. I use asterisk sip trunking for small businesses, so my setups are pretty basic. I am atempting to switch over to TCP so I can offer service on ios devices. I can make a call on the 550's when they first boot up, but let them sit and they become unresponsive.
I am using zoiper on my android phone, without issues via tcp or udp. I'm also using lastest polycom sip software: 4.0.10.0689