Perhaps because we sometimes miss the obvious because of the obscure?? Very well could be, although that didn't seem to be the question. (It also wasn't clear whether it was the mother or the caller who was going to be the signer... )
Assuming it was the mother's signature that needed to be notarized, the fact that she was unable to leave home would seem to be an appropriate use of credible witnesses, if allowed under IA law.
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