Talking on a conventional telephone receiver limits your movement by forcing you to hold the receiver against your ear. It also means that if you want to type or take notes you are forced to cradle the handset between your head and shoulders. This unnatural position can lead to muscle stress and even back injury. This is because while your body is in this position the muscle system which supports it, tends to contract and shorten. This in turn causes a reduction in the blood flow inducing persistent pain, which then causes further contractions and so the vicious cycle continues...
Another consequence of maintaining this bad posture on an ongoing basis is that the constraint of muscles in the neck and head can cause recurring headaches. Guaranteed to impact your staff efficiency...