I have a customer with about 10 in use and they like it okay, but if you have users that don't have a need for all the BLF's and just want an app to use for dialing out and receiving calls, check out the NEC ST500 App. It's free, and just uses a standard user license. It does communicate on port 5070 like the 3rd party sip. Make sure you have a password setup with authentication if you are going to open up 5070 to the outside world.