An Agent will use the phone to call you if he knows he can't get a warrent. When he calls back ask your friend to get his phone number and they will pass it along to you. You then return his call from a safe place, like a gas station in the middle of nowhere. Be polite and as helpful as you can without being a snitch and don't admit to anything. You are just being a good citizen and are returning his call. If he asks you direct questions about AAS tell him you want to cooperate but you feel you should consult with a lawyer before answering any questions regarding any illegal activities. Don't say regarding AAS say regarding any illegal activities.
The key to all of this is you always give them something. They want to talk so give them a talk. They will feel happy that you called because they can put a paper in the file and it looks like they are working. But don't give yourself up just try to make them happy with a phone call.
It's a lot like calling a girl after the first date. It doesn't mean you are going to go out with her again but it does make her feel better.