my question is I called my friend on a whatsapp, and his phone said "on another call" and did the busy tone, twice. I then called him again and he said "he was not on another call, that he received a normal call (outside of whatsapp) at the same time and did not answer and they kept calling hi,
Would whatsapp know this and say the person is another call and notify me that he is on another call?
"On another call" is (as far as Whatsapp's concerned) is synonymous with "Line engaged". You'll get that message when the person you're trying to call is actually speaking to someone (irrespective of whether that call is routed through Whatsapp or not) OR when their phone is 'busy' because it's ringing. (The system sees a ringing phone as 'engaged').