People who are deaf or hard of hearing use a variety of ways to communicate. Some rely on sign language interpreters; others use assistive listening devices like hearing aids or cochlear implants; and others may rely primarily on written messages. Many can speak even though they cannot hear. The accommodation your company would need to provide would vary depending upon the abilities of the person who is deaf or hard of hearing and on the complexity and nature of the communications that are required.
So your first question is probably: In particular, what kind of accommodations will I have to provide? And how much will it cost? The answer to that question will depend entirely on the accommodation needs of the individual, and the deaf or hard-of-hearing person is your best resource.
Luckily, thanks to modern technology, communication between employers and deaf employees is easier than ever - with e-mail, instant messaging, chat programs, texting and paging. Even the telephone is no longer a barrier because of the ability to use relay services.