Shoulder rests (pictured to right) are not recommended because the adhesive typically causes damage to the phone equipment. Our experience has shown that the handset (worth about $100) is damaged beyond acceptable use to someone who inherits the phone after the person currently using the phone applies a shoulder rest to it.
Please do not adhere anything to your telephone as it causes the phone to have a shortened life expectancy.
Instead of a shoulder rest, we recommend that you consider a headset option. A headset is much more ergonomic for you than the shoulder rest.