i have done a lot of testing in this area, dial-up over voip works but yeah, slow speeds, the max i did reliably working was 14.4k speeds with linksys ATA that have the "fax" option, if that is what you refer with #99 then yeah it will work with any number, it just set the ATA (the device that converts phone to Ethernet) in fax mode that changes some settings (disabling echo suppression and stuff like that) and codecs.

I won't be doing dial-up internet, just dialing BBS's and other computers since the machine I have has components from 89/90/91 and therefore before the age of the internet, and I would like to keep it that way.

Based on my experience (working for a VoIP operator) it's definitely doable if your provider is using a PCM (or equivalent lossless) codec, such as G.711. But even then, 14.4k is about as fast as you can get reliably, but it should be reliable (and especially at lower speed than that like you are likely to find with BBS). I have heard of a customer reportedly being successful at 33.3k but I have my doubts. However, if your provider uses a lossy codec like G.729 (which is becoming more popular, as voice quality is audibly more or less the same and much less bandwidth) then... good luck.

What codec your provider uses should be a question they'd be willing to share (or, at least, we would).

An idea is to get an OBi box that does Google Voice. That's my plan as I don't want to tie up the phone line for a modem. You can also go hunting around for a system that has a proper lossless codec.