Weird emergency call bluetooth problem

Bit of an odd one this so seeing if anyone else has encountered it.

Last week I needed to dial 999 from the car (scroats throwing things from motorway bridge   ). iPhone 4s handset connected to retrofit MDI. Tried to dial from the MFSW using the 'dial number' option. Call failed and bluetooth disconnects. Reconnect, try again with 112, call fails and bluetooth disconnects. Disconnects from MDI, try again and still fails/disconnects. Dial from handset and select output via BT, all works. Forget and re-pair, nothing changes.

Check phone logs and only call from handset show. All other calls, including dialling 121 for Voicemail work so doubt it is handset and not network.

Now I don't expect everyone to dash out and try calling 999 (really, don't!) and it's going to be a bugger to test.

Any ideas?