We have a problem with the Vtech 2511 sip room phones calling the mitel ip phones. (front desk, restaurant, etc). The phone in the room will go right to speaker and there is no way to take it off speaker mode. Mitel and Vtech engineers are working on this but have not come up with a fix as of yet.
If anyone has a specific model of third party phone sip phone that is installed on the Mitel 400 that seems to work ok please pass the information along.