AT&T service?

I do have unlimited internet access and e-mail with AT&T and paying $20.00, called AT&T and asked what would happened if I get an AT&T ( HTC 8925) unlocked phone , they said our system will recognize it and will lock the system and you have to pay more because HTC 8925 is a business phone with PDA, basically more data transfer, and I was wondering if somebody knows about it.