You would be surprised. I believe that all mobile phone calls are in fact being recorded.
I had a "voicemail" once that dropped into my mailbox, that was actually a recording of a phone call that I had with someone, that did NOT go to my voicemail. So to me, that's proof-positive that they are in fact recording everyone.
That could just be a one-off fluke where the system glitched and thought a phone conversation was a voicemail message. I had a weird experience once where I dialed a friend and somehow got connected to another conversation he was having. He couldn't hear me, and I couldn't hear the person he was talking to, but I could hear him. It took me a while to figure out what the hell was going on. When I called him back later he said that he was on the phone with someone else and that my call never showed up on his end.
Weird bugs happen all of the time with software. Also, the amount of storage that would be required to record every phone conversation would be staggering. Not to mention the legal ramifications if that information ever got released. Also, the number of people that would have to know about it would be so staggeringly high, and the violation of rights so heinous, that there's no way it could be kept a secret.
If the government really wants to know what you're saying they'll just get a warrant and then start recording just you. Otherwise everything they have would be inadmissible in court anyway.