Mr. GMAlso hey, note for those playing at home:
Enabling IP masquerading may or may not be necessary, based on my loose understanding of dial-up. If it is needed, Debian 13 uses nftables. Changing stuff in nftables involves using the "nft" command that does nothing if you aren't root or add "sudo" beforehand. I was tripped up by this until I stumbled upon a post online saying "hey use sudo first" and it suddenly worked.
Additional note: Alpine Linux does not have a working version of the mgetty command. Trying to install it does not work. I started on Alpine Linux and moved to Debian once Alpine didn't work, and then I spent the day figuring out Debian's idiosyncrasies.