Dear all,
I recorded a call on my mobile and one voice is loud and clear and one is very inaudible.
I tried to equalise it but it made no difference.
Audacity 2.3.2 and Windows 10.
I appreciate you in advance for your advice.
I don’t think that’s corrected with the equalizer. Anyway, you probably mean “compressor.” I don’t know how to use the compressor. It’s probably the simplest method. Otherwise, I would just boost all of the weak parts with Amplify in Effects. Use Control + R to quickly repeat the boost at all selected parts.
I recorded a call on my mobile and one voice is loud and clear and one is very inaudible.
So the local caller is loud and clear and the far side is unusable?
The far side will probably stay unusable. That’s not an actual voice. That’s the phone’s environment processing and echo cancellation error. It’s honky, garggly and fades in and out, right? I think you’re stuck. There is no good voice there to rescue.
There are special programs which can record both sides, but those can be an adventure because many countries have laws against clandestine recording.
Koz
LevelSpeech2.ny (compressor+limiter) plug-in for Audacity could possibly save the day …
Trebor and LevelSpeech2.ny have absolutely saved the day! 
Thanks you SO much!
Thanks Trebor.
LevelSpeech2 was the perfect solution!