The Disney stars have been close pals since meeting at an audition for children's TV show 'Barney and Friends' and Demi - who entered rehab last November for three months - revealed Selena kept in constant touch throughout her treatment.
She said: "I think I was out of tears at that point [Selena] called me and she was crying, she was like, 'Hello.' I'm like, 'Why are you crying? I'm the one in treatment - like, I should be crying'.
"She's like, 'Are they treating you OK?' I was like, 'Yeah, they're treating me great.'
'She's definitely one of my closest friends and no matter what, we'll always be close as sisters.
"We met 12 years ago, that's more than half of my life."
Demi was treated for an eating disorder and admitted at the height of her illness, she was making herself sick up to five times a day.
She told 'Access Hollywood': "I think at the worst it was a little more than that. I was just so sick. I thought that Orange Juice was going to make me fat.
"I feel amazing. I feel really happy and healthy. Life is beautiful and I can't complain."