If one (during salaah), when standing, reaches down to the ground and looks what time it is on the mobile phone (to see if one prayed one ruku or rakah in time) - is one’s prayer accepted? (The person doesn't remember exactly which prayers and how many they were)

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

If a person bows down in his prayer until he reaches the position of Rukoo’ [bowing down], then he does something additional, this invalidates the prayer if he has done so deliberately while knowing the ruling. However, if he does not know the ruling, then his prayer does not become void by doing this.

Indeed, this question was asked to Shaykh Ibn ‘Uthaymeen : “Shaykh: A man in the Thuhr prayer dropped a tissue handkerchief while he was standing in prayer, and he bowed down and took the handkerchief, would his prayer become void by this movement?” The answer was: “Yes, his prayer becomes invalid by this movement because when he bowed down he reached the position of Rukoo’, so he added another Rukoo’, but if he did so while not knowing the ruling, then he is not required to do anything for the general saying of Allaah (which means): {“Our Lord, do not impose blame upon us if we have forgotten or erred.} [Quran 2:286]Therefore, if you drop a handkerchief or a key while you are standing in prayer, you should leave it until you reach it in the prostration position.”

Based on this, it is not an obligation for that person to repeat the prayers in which he had done this act if he was ignorant of the ruling, but if he knew the ruling, he should make up those prayers to an extent that he is certain or predominantly thinks that he is acquitted from liability (i.e. that he has made up all the prayers in which he did this act).

Allaah Knows best.