R. Kelly Under Investigation for Possessing Retired Warden's Phone Number

R. Kelly is under investigation for possessing a retired warden's phone number at his federal lockup in North Carolina ... and he's been placed in isolation for the duration of a probe of the incident, according to his lawyer.

Kelly's attorney, Beau Brindley, tells TMZ ... the R&B singer's problems began when he got involved in a mentor program to help guide other inmates, which was overseen by the warden at FCI Butner Medium 1.

Recently, the warden retired ... but before he left, he gave Kelly his cell phone number in case the convicted felon needed to reach out to him to seek advice about the mentor program.

Some time after that, Kelly's cellmate got busted with a mobile phone on his person -- which is considered contraband in prison.

The lawyer says prison officials searched the inmates' prison cell and found Kelly's notebook, in which the retired warden's phone number was written.

Prison authorities placed Kelly in the Special Housing Unit -- better known as the SHU -- while they investigate the circumstances surrounding the former warden's number appearing in the notebook, Brindley said.

We reached out to the federal Bureau of Prisons ... a rep tells us, "For privacy, safety, and security reasons, we do not discuss the conditions of confinement or internal security procedures for any inmate or group of inmates."

In 2021, Kelly was convicted in New York of racketeering, bribery, illegal sex with minors and other federal offenses -- and was sentenced to 30 years in prison.