Firstly, a deleted answer is only visible to logged in users with more than 10k reputation (on graduated sites), so most visitors to the page won't even know it was there.
Secondly, if a post has even one spam flag on it then when it's deleted it's hidden behind a "this post was flagged as spam or rude" message so even 10k users won't immediately see the phone number. They would have click through this message to see the actual post.
Thirdly, it's spam. As others have pointed out redaction is there to protect the innocent from their own mistakes. We don't owe that same consideration to spammers. It also takes two moderators to perform, so it's not a cheap operation.
Fourthly, as it's spam it's critical that it's left alone and not edited so that the various spam filtering tools work correctly. If you edit a post that's marked as spam so it no longer looks like spam then it will be useless as a review audit (for example).
The only exception I'd allow would be if that personal information was known to be someone else's and was included for malicious purposes. You would need to make that clear in your flag requesting redaction though.