The contact telephone number for BAH should be on your non flight voucher. If you are travelling or within 2 days of departure the duty office is open 24 hrs and you can reach this team by selecting the relevant option. If you are outside of 2 days or not travelling I think it’s open 8am to 8pm uk time.
If you select the option for the 24hr duty office and you are not away or more than 2 days before travel you will most certainly be told to call back during office hours to enable this small dedicated team to deal with more pressing matters.
For the reason thst they are extremely useful when away such as in irrops irrops it’s a good idea to save the BAH number when you make the booking.