I use captel for my iphone.
I do not use it at work so I have no experience there.
I feel it works pretty well on the personal level.
Works well when I need to call the doctors, make appointments and such.
I do explain that I am using captions, so they understand the delay in some of my responses.
BTW, I have also tried ClearCaptions which is also alvailable for the iphone, but I feel Captel is much faster, the delay time shorter, and although there is a slight problem with the spelling at times,
all in all, it is a better service.