My guess is that your HR people don't really want you consulting over the phone, either. That puts a bit more sense onto it, once you think about the problem of telling who's disabled and who isn't.

Suppose you could do X% of your job at home. Then it would be tempting, presuming the costs of disability insurance work out right, for your company to keep you as a "disabled employee" for as long as possible, since they're getting a percentage of you for a fraction of the cost.