It depends on your aids. Mine have no phone switch (Widex BTE with RIC - nice aids, if a bit pricey), so it has to be a handsfree that fits over the aid comfortably. i've always used the Motorolas - I'm on a 720 now. Almost loud enough in the car (loud enough if I press the headset tighter to my ear) and comfortable enough to wear all day. (I have to talk to Widex about putting the mic in the receiver, though. That would make the aid ideal.)
With my old aids, with a phone loop on the aid, the Motorola worked through the aid, but unless I was clear of civilization it picked up hum from the powerlines almost everywhere.
If money is no object, there are aids that have Bluetooth in them, so you just pair the aid to the phone and you're done. And you get the full correction for the phone call. (They pick up from your mouth as good as any headset.)