i take my phone to the bathroom and place it on the counter and sometimes i touch it when i’m in the bathroom but i can’t remember if i touched it with najis hands or not but i do not see any sort of sign of impurity on my phone, i sometimes pray with this phone in my pocket or with this phone in front of me if i forget the last part of prayer and my head touches it is my phone najis and my prayers invalid
Let me tell you in the most simplest way I can.
First of all, whenever you take your cell phone to the bathroom I'm sure you do not touch it with your hand right after you have cleaned your waste with the very same hands.
Secondly, just taking anything with you in the bathroom doesn't make it impure. If that was true then we would be instructed to also change our clothes and scrub ourselves thoroughly every time we come out of the bathroom.