Is there a rule against using your cellphone while pumping gas at the Sam's gas station? Is there a sign posted?

Yeah there is a sign on almost every gas station ive been too, including sams

Yes and yes. There is at most gas stations. Unlike many gas stations however Sam's club employs an attendant at all times to assist people and maintain safety polices. Such as cell phones while pumping or filling up gas containers anywhere other than the ground for example

Even if it’s not advisable to use a cell phone while pumping gas, you must remember: This is Sam’s Club. If anything happened, it would be an improvement.