A status of "not in BOP custody" with an unknown release date does not mean he has been released. It means he is in custody somewhere that is not a Bureau of Prisons operated facility.
This status typically appears in a few specific situations. The most common is when someone is being held in a county jail or a US Marshals holding facility awaiting sentencing, designation, or a court appearance. Until the BOP formally designates an inmate to a specific federal facility and receives them into custody, the BOP locator reflects this transitional status rather than a current location.
An unknown release date alongside that status usually means sentencing has not yet been finalized. If there are unresolved charges, a co-defendant whose case is still pending, or any other reason the court has not handed down a final sentence, the BOP cannot calculate a release date because there is no sentence to calculate from. He may be in custody awaiting a hearing, a plea, or sentencing on an outstanding matter.
Once he is sentenced and the BOP designates him to a facility, the locator will update to show his current institution and a projected release date based on the sentence imposed.
To find out where he is right now, check whether the US Marshals Service inmate locator shows his location since Marshals often hold federal defendants during pre-designation periods. Contacting the court clerk in the jurisdiction where his case is pending can also tell you whether any hearings are scheduled and what the current status of the case is.
He is in custody. The communication gap is almost certainly a function of being in a transitional holding situation where phone access is limited.