VoIP.ms Is a great inexpensive hosted solution. Peding what your firewall setup is, you could buy Nortel 1140e and 1120e sip sets (I think these are the best looking IP phones on the market and load the sip 2.2 free unlicensed sip firmware into the phones via free downloads from avayas website and log each one into VoIP.ms via username and password. The phones will function almost identically to being on a Nortel switch.
For all of you anti Nortel guys, Which seems to be plenty of people today ditching perfectly great working Nortel equiplent to go with Cisco because they are "cool", you can use Cisco, Polycom, lg, yea link, grandstream or virtually any sip phone on their platform.
Back to the Nortel sip phones, You can use later firmware editions, but
You need to modify the code so that the sip firmware looks for basic sip operating features, and disable the extended feature set of Avaya features.
One thing you must ask VoIP.ms
To activate is the call transfer feature. For some reason they leave transfer off until you request it.
Let me know if you have further questions!