Unfortunately no. Mitel, as well as HP and Windstream had no answer. However, I have an idea, but I have to talk with my vendor first. The only things that changed when this started was in the adding of the SIP trunks and an MBG. The MBG actually does not filter the SIP trunk due to the Windstream configuration. So we use the MBG for simply doing MiNet and SIP phones (UCA app).
The default route of the Mitel 5000 had to be changed to point to the Windstream router to allow calls to be forwarded to Windstream. So now our default route, which was our HP stack becomes a secondary route on the Windstream router. So, I am guessing that the DHCP on the Mitel 5000 has also changed the default route from the HP stack to the Windstream router. So when a phone call is initiated, it has to bounce off the Windstream router before getting back to the Mitel switch. In reality, this should not be an issue, but as for what I am seeing, the insufficient bandwidth mostly appears (I say mostly since I have no hard facts to say all) when a phone is using a SIP trunk (i.e. inbound/outbound call). I can make a "internal" call to our plant 3000- miles away and talk for 30 minutes and never get a insufficient bandwidth message. If they receive an inbound call, they may get it in a a minute or so.
This leads me to still center my attention to the network configuration of the Windstream router and the Mitel switch. However, I do not know how to reprogram the phones to point to the internal HP stack as the default route. (the phones do not need to the Windstream router to be their default route)
Option 6 in the IP Settings >DHCP Server Settings > Options is Option ID 3 (router default gateway) but my configuration reads as follows:
Scope = IP Address Range and points to IP Settings >DHCP Server Settings > IP Address Ranges > 1
Format = Mitel Automatic (can I change this to IP address??? and force the local stack...)