How do I stop my iPhone 6S blocking me signing in with a different Apple ID

Hi,

When I purchased my iPhone 6S for work, I logged in for the first time with my work Apple ID & Password.

I now want to log into my iTunes Account on the iPhone using my personal Apple ID & Password, but the phone keeps blocking me stating my Apple ID & Password are incorrect.

I am logged in to other devices with the same Apple ID & Password, and can successfully log-in using my families log-in details.

I can only think that somewhere on the device, my personal Apple ID is saved somewhere and creates a conflict or ck when I try to use it.

Hope someone has an idea how to solve this issue?

Thanks.