You should ALWAYS assume they are listening to you. But your conversation and contacts can stay out of their reach when you use Encrypted apps like signal - By the way, Signal is a real huge "data hog", so if you use it with Cellular data, it can eat up 1.6 gb/hour. My MONTHLY plan is only 2 gb a month. So I use the voice part only when on a WIFI. The Text portion leaves a much smaller Data footprint of course, and unlike the voice part, it is not blocked from China. But I heard Signal has an upgrade (only for Android) that the great firewall of China cannot block. Wickr works from China. At least in the parts where my friend visited.
RE: Switching System 7 (SS7) Security flaws