We use/have resold the Bandwidth.com service. Really have no complaints with it.
The only thing with SIP trunks is that the better internet connection you have, the better it will sound/work. In a typical Bandwidth.com setup, you won't need to use the MBG server at all. As part of your service with them, they will supply you with a SIP aware firewall/router (Typically a Edgewater brand router). They will configure the Edgewater for you, for both QoS over the internet and redundancy to their servers. Then you simply have to plug it in ahead of any other data routers or firewalls that you have, and program the Mitel to talk to the inside address of the Edgewater.
- This config will then only cost you SIP trunk licenses on the 3300, and not the additional SIP trunk licenses that the MBG server requires.