Taking rest breaks during the day and even a short nap in the afternoon can be a great help. It is painful to hold the phone receiver during long calls. A simple solution may be to attach a shoulder rest to the phone receiver. Another possibility is to use a speakerphone. If you use the phone frequently, you may wish to consider using a head set.
Repetitive tasks are very tiring.
In some cases, you may be able to change the task or take short breaks while doing the task. For example, if the problem is grading papers, you may be able to assign shorter or fewer essay questions and use more multiple-choice tests that can be machine read. Another solution is to request an assistant to help with the task. It is difficult to work a fixed schedule. Talk to your employer about having some flexibility in the hours or days that you work.