There’s only one message on the voicemail of Dr. Jon Hallberg that is sacrosanct, never to be erased. It’s an improvised birthday serenade that singer/actor T. Mychael Rambo left for the Twin Cities physician.

Hallberg cherishes it.

“It’s a gift — like a painting or great photograph,” said Hallberg, who has known Rambo for years and has hired him to perform. “I play it from time to time and it’s impossible not to smile while I’m listening to it. The doctor does a little self-healing this way.”

Rambo may be a Twin Cities celebrity who just ended a successful run as the blustery title character in “The Wiz” at Children’s Theatre. But he’s a grounded figure who does lots of big-hearted things to uplift his community, from hosting Father’s Day celebrations that highlight active, engaged dads to teaching and mentoring kids.

Singing his singular versions of “Happy Birthday” to friends, family, co-workers and strangers is part of his service. These serenades, which last from 15 seconds for squirrelly kids to three or four minutes for swooning adults, are delivered via phone, through social media like Facebook Messenger and sometimes even in person.

He sings for recipients young and old, of any gender or ability, three or four times a week, often selecting from Facebook friends but also birthday partiers he may happen upon in a restaurant or mall.

“All the things that divide us melt away with song,” said Rambo. “Music is something that heals us.”

Strangers often are suspicious at first when he starts to ask them questions about themselves and their favorite things to get a sense of their personalities to distill in song. When he starts to sing his birthday greetings, they think that they are being pranked.

“They always look for hidden cameras,” he said. “And after a while, they start blushing. Then they want to pay for my meal or send over a dessert. Of course, I don’t do it for any of that. Singing for me is like breathing. It’s something I do to live.”

Twin Cities-based sex-abuse expert Cordelia Anderson has received many birthday songs from Rambo over the decades.

“I get up at 5:30 and he usually calls early in the morning, so his glorious voice is the first voice that I hear,” she said. “It’s one of the most beautiful, heart-warming gifts anyone can get. It’s personal, it’s individualized and it’s T. Mychael Rambo.”

Rambo sees these personal songs as an extension of the grace that he has been shown in his life. He’s battled addiction and homelessness. He’s fought through personal tragedies.

More than 20 years ago, his mother was struck by a distracted driver in Oakland, Calif., where she lives. She was left paralyzed and without the ability to speak. When Rambo sings to her, she lights up.

“It’s almost like we’re communicating at a spiritual level,” he said. “It reaches people on levels that are beyond thinking and resistance.”

Hallberg, the doctor, said that hearing Rambo’s rich and warm voice produces a physical change in him. “I’m sure you can measure it and get quantifiable data,” he said.

That Rambo is even known as a singer is a part of the grace imparted to him, he said. An Army brat whose family seat — Rambo, Texas — was founded by freed slaves, Rambo did not have much of a voice as a kid. He never sang in choir or glee club, “but I sang around the house, and loudly, much to the annoyance of my family, who just wished that I would be quiet,” he said.

Then, at college, something clicked with his voice, he said. “I heard myself one day and I had good tone, sweet melody.”

Others heard it too. He was invited to sing lead for a band called Three Mikes and Chord. “Three of us were named Michael and one was Chord — Chord Jeffreys,” he explained. “We didn’t last because we weren’t committed to it and I was struggling with alcoholism.”

That struggle is what brought him to the Twin Cities, for successful treatment at Hazelden. Piecing his life back together, he started working at a yogurt shop in the St. Paul skyway, where he sang ebulliently. The Minnesota Opera’s artistic director, Dale Johnson, heard him and encouraged him to audition for the company’s production of “Showboat.” He got a part, and a career was launched.

Even though Rambo does not seek any money, he is an artist, and those who appreciate his talent have sought to remunerate him. Serena Wright, director of alumni relations at the University of Minnesota, has hired him to sing for commencement but also to wish her friends happy birthday.

“I value his craft, his talent, and all the work he’s put into it,” Wright said. “I think part of what’s so special about him is his ability to first listen and take things in. His life could’ve gone an entirely different way and now that he has this opportunity to spread love, he’s doing that. And being lifted up by his voice, by his ability to touch you to the core, is a beautiful thing in a world where a lot of things can get you down.”