I believe this is the phone system that calls to confirm your doctor's appointment either at Bolling AFB or Andrews AFB. Maybe it is voice activated, so if you don't answer, it does not leave a message. There are probably privacy rules related to HIPPA that say they can't leave a message saying they are confirming your appointment at mental health or wherever! If you receive this call in error, it is probably because someone has fat fingered their phone number in the online appointment calendar system so it is calling you instead of them. These are my guesses!
This might be a US military member using a calling card (from their home state) or whatever, trying to call friends and family located in the US, while they (the military member) is stationed in Iraq etc.