I use to have the same set up UA5>NJb3 and I remember in '05 at HSMF my recordings started like this and got worse because I had a bad optical cable and plus I used those right angle adapters. I got a new optical cable w/ built in right connector to my NJB3 and all the digi noise went away. I asked a electronics geek friend of mine about cell phone interference once and he said just like already pointed out that w/ proper ground this is not a issue.
PEace