Need more info about which BT module you installed and/or from which model BMW did you get it from.
There are several types...the early BT modules were "accessories" for the e38/e39/e46 models that were built prior to 10/2004...and are usually referred to as ULF. After that, the BT modules became standard equipment (not from the BMW accessory catalog) and are referred to as TCU...these modules don't use the "phone button" to pair...it will be open for pairing up to 2 minutes AFTER turning the ignition on.
I have an 06 with the later BT TCU...and to delete previously paired phones my manual says:
1. Press & hold the call/talking face button (the one with the face profile on it...see attached pic with the button highlighted), then after about 3 seconds turn the ignition key to position 1 and release the button after approximately 3 seconds. "BLUETOOTH PAIRING" appears on the display or monitor.
2. Press the button again for approx. 10 seconds until you hear a triple acoustic signal. "PAIRED DEVICES DELETED" appears on the display or monitor.