Yeah... We saw this start happening at around release 7 (guessing, its been awhile)... although dial-up data has always been kind of flaky over IP, it seems in the last few years almost any analog, dial-up data device is not going to work across IP trunks EXCEPT faxing and then only if you have DSPII cards and use T.38
I have tried getting various dial-up modems, credit card machines, alarm systems (DTMF based ones work fine), water meters, time clocks, etc. to work over IP trunking and it fails 98% of the time, it fails so much that I quit trying it.
This is not unique to the Mitel, in our hosted SIP side of the business we have a rule of thumb, if it has dial-up data and not a fax then it has to stay on an analog CO copper line... a lesson learned the hard way.