Ya estás allí
You are already there
¿Ya estás allí?
Are you there yet?
It would be wrong to say 'Ya no' when trying to mean 'Not yet'. The translation of 'Not yet' is 'Todavia no'.
However, 'Ya no' is a very popular phrase used to say 'Not anymore'.
So the word 'Ya' means sometimes 'yet', sometimes 'already', and sometimes 'anymore'?