I have a bare bones voice recorder app I use called Tape-a-Talk. Simple, easy to use, records .wav files. I use it to record conversations, like at the doctor's office. I ended up running the recordings through Audacity (an audio editor) on my PC to amplify the recorded material and convert to mp3 files.
I have also used one called Smart Recorder that had the ability to skip moments of silence and only record sounds. I used that one when sleeping to see if I snored, talked in my sleep, or had bouts of sleep apnea. (I had none of those things but I did make some, um, unflattering noises.)
Neither of these apps would record phone calls. If I wanted to record a phone call I used a landline on speaker phone mode to make the call and placed my cell with Tape-a-Talk voice recorder running next to it.
You might search the Google Play store for "voice recorder editors choice" and see what pops up.