Some use frequency hopping techniques to spread the signal over a wide range of frequencies. If the hopping rate is slow enough, you may be able to hear it in your hearing aids. This is a big problem with non-digital hearing aids, but digital ones are supposed to be less sensitive to the problem. With non-digital aids, the interference sounds like a cat purring, or a helicopter hovering. All hearing aids may whistle when you put a flat object like a deck of cards up against your ear.
Some cell phones are flat, sort of like a deck of cards.
Regular phones usually have an ear cup, which is much less likely to cause the whistling. Now the Widex senso digital aid is the only one that I know that has an active feedback preventing circuit. When it senses feedback, it reduces the gain at that frequency and gets rid of the feedback. It needs a few seconds to do this however. So, if that is your problem, and if for some reason your audiologist can't get your aid to fit better, just hold the phone steadily against your ear for a few seconds and the feedback should quit.