I have a case open in Tier 4 right now with this hoping they will fix it. If you add a login code under the user's telephony tab, you get a "Failed to register for VoIP operation" error when logging in.
You can switch between a soft phone and hard phone no problem by logging off the hard phone extension before logging in to the soft phone, which isn't a great solution if the customer forgets to logoff before leaving their office - and there is also the issue that if you have G711 hard coded to the VoIP extension for internal calls, you have to use G711 on your soft phone remotely unless you remote in and change the config - not customer friendly.
But in every test from 3.2 to 4.1, if you add a login code and hot desk the user, VoIP registration fails.