Yes.Not by itself, I would think. I assume your cell phone bill is not over $50 per month which is not considered very much debt load. Yes it is a month payment, but not a lot to pay. Having a credit card would be better to improve your credit score than just a cell phone bill even if it is not a major credit card and just a dept. store card.
I think having a dept. store credit card help me get a major credit like Visa or Mastercard which you can use almost anywhere. I do not remember seeing the name of a cell phone company on my credit report many years ago. I paid about $45 to 50 per month and I am in Canada so if the other guy talks about Sprint and AT and T which are American cell phone companies which the rates are even cheaper I believe. I pay more and my bill with the phone companies was not mention in credit report.