I really don't know guys if you haven tried this, but believe me ...
If you define a SIP trunk with ANY PBX (I mean, 3CX, Asterisk, even a Cisco router), you will can to call to ICP-3300 without troubles...
All this, without define a SIP trunk on the Mitel... you just have to define it on the PBX that you wanna make the call...
Have you even try this?
AH! I have tried it on Mitel 3300 Rls. 7.1...
Saludos!