I know that the amount of mailboxes is limited to your configuration. For example, you have 15 phones and 15 mailboxes. What if you need 10 guest mailboxes on top of that. Can you purchase licenses for those?

I may be wrong, but I think all you need to do is create those guest extensions in Users. In the telephony tab of each of the guest extensions, put in 123 in the login code field. Save and merge.

Yes, you can use phantom users with mailboxes.