I think part of it's due to the perceived privacy i.e you can 'text' at anytime without bringing any attention from surroundings, but that's not possible with '

I think part of it's due to the perceived privacy i.e you can 'text' at anytime without bringing any attention from surroundings, but that's not possible with 'calls' when you are in the office or when surrounded with friends/parents/relatives etc.