I just dont see it picking up interference from a phone as I dont think our cell phones run on or send any frequency 80hz and below (I have no scientific proof of what frequency our cell phones send signal out as.) It is basically the same reason why you dont hear alternator whine from subs.
What is your freq set on? What kind of rca cables do you have. The only thing sending any kind of actual signal to the amp that would cause the subs to play anything other than bass from the music is the rcas. Getting some better rca cables would probably be the best thing for your system (chances are that would probably get rid of that problem), if it is in fact picking up interference from a cell phone.
I can only speculate because I have no proof of this not being the problem but out of almost 10 years experience in car audio, your case is the first I have ever heard of having this specific problem. Sure I have heard of it in computer speakers and sometimes really cheap home systems but never car enviroment and thus the reason I have a hard time believing this.
