I just found this interview with his engineer Sean Paine http://www.hotnewhiphop.com/meet-gucci-manes-middle-man-sean-paine-news.17941.html
He mentions he records verses thru the phone also.
Sean: Yeah he calls, like once a month, just to check in and see how I’m doing or whatever. We talk besides music, and then when it’s something important he might call and record a verse, I might record a verse for him.
HNHH: Okay yeah, so some of his new collaborations, the recent ones, some of them are recorded through prison and some [are] old?
Sean: Yeah some of them are recorded through jail, he calls on my phone and I’m recording it.
HNHH: Is the quality there? Can you tell that it wasn’t recorded right in the studio?
Sean: Nah, I mean I kinda engineer it so you can’t tell that it’s recorded in jail.