Call forwarding not turning off

I have had " forwarding turned on " for a while now and went to turn it off to begin to receive calls as normal on my work phone. When trying to do this it will not turn off. I have done everything that I can think of and I am now down to it is either the phone itself of the carrier. In my research I have found that it is a issue among many carriers and am trying to find my next coarse of action. I changed "SIM" cards that is a different carrier and the phone turned the "call forwarding OFF". I put my original "SIM" back in the phone and went right back to the same issue with the call forwarding.