Most usually it is a hard inquiry when a cell phone company checks reports.
I am not certain they even worry about scores.
I think they look more at your debts to see if you have any prior phone/utility bills that were sent to collections.
I had higher scores than my spouse, I had to have a giant deposit for cell phone, but spouse got approved for up to 5 lines with no deposit. Spouse had more collections, but had no utility/phone collections. I had a bogus SW Bell account, that has since been removed.