Pull the plug
If your phone is tapped at the source, there’s not much you can do about it, and you’d never know it until you heard the recordings at your trial, anyway. You can, however, limit its utility as an active listening device.
Keep an eye on your battery. Is it draining unusually fast? If your phone is sucking more juice than usual, that may mean it’s being used as an active listening device. Constantly transmitting audio to a third party requires a great deal of your phone’s resources. Ruin the government’s day by removing the battery when you’re not making a call. If the microphone has no access to power, it can’t listen to you. This won’t prevent the NSA from hearing your long-distance arguments with your mom, but it beats having some dude listening to you in the bathroom.