[Users] Accounting wrong BYEs

Hi all, I have a problem with accounting wrong BYEs: I have noticed that some UAs like the spa are sending BYE to cancel an INVITE instead of a CANCEL message. Now openser is interpreting this correctly, but the problem is with accounting which will account this BYE as call stop. Since there was never OK to the invite, there is no call start and just a call stop which makes my billing very unhappy. Is it possible to have something in the accounting module that will prevent such "fake" byes of being accounted? I am also thinking how to script it in my config, what i have thought of so far is when receiving a BYE to check if there is ongoing transaction and if there is, do not flag the BYE for accounting. This however will probably result in some real BYEs not being accounted too, because (correct me if wrong) the transaction will still exist in mem for some short time after the OK is received. Any help or suggestions will be greatly appreciated. Best, Dimo