Probably requires dealer programming. Since you don't have a subscription, it is not active. I am not actually sure that you really turn it off in the TCU module through dealer programming, but you could call your SA at the dealer and ask them. If it can be done it would require a BMW computer, some independent shops that specialize in BMWs have it, but not many.
Still, the button sends a command over the iBus and the command should be seen by both the Assist TCU and the Stealthone. Does the V1 mute when you press the button?
You could also try removing (or disconnecting) the TCU unit, assuming you don't have a version with bluetooth (and use the bluetooth), with an '04 car you would have had to replace the original TCU with a bluetooth capable one.
2005 M3 Silbergrau Metallic on black; 6MT, ZCP, ZPP, ZCW, Nav, Dinan exhaust, two AR20 resonators in section 1, Autosolutions SSK, Fucy CSL diffuser, Intravee II, illuminated ZHP shift knob, stealth V1 install with ash tray remote, OEM first aid boxes under both seats, Ultimate pedals and more...