I would agree on Bria as well, although I have played with Media5-fone successfully as well.
On the 3300 you need some kind of SIP-aware firewall/router, SBC (Mitel Border Gateway), or a VPN connection.
And you do not need SIP Trunking licenses, just an available Teleworker user licenses, and a little config on the MBG since SIP is disabled by default.
Now, on the 5000 it is SUPER easy, a little port forwarding in the router and away you go.