How to minimize texting /calls without making them think I'm disinterested?

I have the exact same problem,especially as i have exams and need to revise but get distracted by people texting me.you could say your running out of credit and have no money to top up-i cant do this becasue everyone knows i have unlimited texts ,woops :/ Or 'im going out now so i have to go,im so sorry,see you later :)' that way it deosnt sound too rude like your disinterested. others ive used are 'sorry i fell asleep' i even pretended my phone was broke for a while during exams!