Until one reaches the mikveh, he/she is not a Jew just yet. Since they are obligated to break Shabbes in some way, can they use phones during the day? Should they refrain?
Welcome to MiYodeya! A person in a process of conversion still has the status of a gentile, and he is obligated to break shabbat. But the best thing is to keep it minimal. In my humble opinion, and how i learned from my rabbis as i am also in the process, breaking very small rabbinic melacha would suffice. When a person keeps on doing something he gets used to it. If you keep on breaking big melachot, youll get use to it and have it difficult to stop doing it when you are fully obligated to guard the shabbat. But if its a small thing that you're breaking, you will be able to leave it easily. Mobile phones are very addictive stuffs, and youll have a really hard time getting over it once you fully convert.