I wouldn't call rewriting a phone number and a key point on your @Calls list duplication of work; I call it a best practice. It will boost efficiency when you're making calls by preventing you from hunting for the number in your support materials.

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I highly recommend that you purchase the GTD travel file folder system. I put mine into an accordion style file pouch that fits into a briefcase. There's a folder called "Action Support" in that system. That's where notes and documentation that supports a next action should live until the action is completed.

Note: If you don't carry that support folder with you when you're out and about you'll be faced with actions that you cannot complete because you don't have the support material to do so. If you can't carry the support materials around then change the context to the location from @Calls to where the support materials are (e.g. @Home, @Office).

If you must speak to a person you're calling but get stuck leaving voicemail I suggest you flag it with a "WF" or "LV" or some symbol that indicates that you did call but are waiting for call back. Don't rewrite it, but make the mark in pencil so you can erase it to reactivate it if you don't get a return call in a timely manner and want to call again.