HTC Desire/Android GSM Protocol Issue

I was playing with ASN1 and Supplementary Services over the Weekend. My goal was to provide extended user information during a call setup. So the first step was searching for information of how it could look like, this involved going through the GSM Spec, the 2nd part was wrestling with asn1c to generate some dummy data (as I couldn't find a trace doing that), the 3rd part was being able to generate that from OpenBSC and send it to the phone.

Well, long story short. It doesn't work, the better phones ignore the data I am sending, all the others refuse to show incoming calls, the Phone failing the most is a HTC Desire with Android. First of all OpenBSC has a pretty simple USSD implementation and it doesn't like that, now when you place a phone call (while Android decides to do a USSD request in the background), OpenBSC will close the channel, not do any Call-Control and on the Android one screen shows the call is terminated, in the decoration it is still active, then the wakeup/sleep logic gets confused, you can not hang-up and then the phone is restarting.

The executive summary: Not implementing USSD in your network (we send an error and such) can make Android phones reboot if someone is placing a call at the time android retries to send the USSD query.