Substitutions, indeed. It is technically a monoalphabetic substitution cipher, principally because Anchises prefers that people be able to read the messages rather than do their recreational mathematics with them for months.

Bear in mind that the cipher changes: two of the ciphers have the same substitution numbers; the others are all different. There are two clues within this tag, one staring the reader in the face, one less so, and numerous clues within Howling itself as to how to decode.