I'm not sure what BT module or headunit. Its stock equipment in Mk VI Golf TDi.
I'll do some research for exact details.
Symptoms as follows :
The phone menu sometimes disappears from the dashboard display.
To fix this failure, I learned to disconnect the BT module (under driver's seat), start the engine for 10 seconds, switch off, reconnect the BT module, then restart engine, phone menu re-appears and works as intended.
I call this disconnect-reconnect dance, a "BT reset".