When you report speech, you are describing the meaning of what was said. It common to backshift because the events that the person spoke of in the present are now in the past, and the events that the person spoke of in the past, must have occurred before the person was speaking.
John said "I went to the clothes shop and I'm wearing my new coat"
John said that he'd gone to the clothes shop and that he was wearing his new coat.
It is also correct to report speech in paraphrase
John said that he was wearing a coat that he'd just bought.
But when the person speaks a general fact using the simple present, then that fact is still true when the speech is reported:
John said "five is a prime number". / John said that five is a prime number.
There is no need to backshift, because we understand John to be stating a general fact.
With past tense to past perfect backshifts, you are describing events in the past. You can use past perfect to help establish the order of events, but it isn't mandatory. If John said "I was given a wrong number". You could report this as fact
John was given a wrong number
John said that he was given a wrong number.
you could backshift, but in such a simple sentence, it isn't essential
John said that he'd been given a wrong number.
That last one would be more likely if there was more to the story
John told me that he'd been given a wrong number by the girl he'd been talking too, but he met her in the street and it had actually been a genuine mistake! So they're going to go out again tonight.