First pf all, you do not have to call them 50 times to trade. You can trade with them after the 30th call.
Secondly, you can take as many days as you want to make the thirty calls. You can call them more than once a day, but you can only trade with them once a day.
In short, you don't need to make every call in one day and you can trade after 30th call (if you have beaten the Elite Four).