How can I run voice over existing coaxial cable?

I've been doing home improvement projects for a while and one thing that I've made use of the most is taking advantage of existing cabling in the walls that isn't being used. The project that did this the most was my "video-over-coax" setup, which uses a composite-video-to-cable modulator and a coax switch to show some video output on the whole coax network, for example the security camera monitor. This thing works by tuning into a specified channel on the TV that the coax is attached to.

I want to get access to my landline phone signal from my garage/office. As of now, there are only a couple of connections from the network closet (where the main networking stuff is, most importantly here the phone line switch) to the garage, which are a coax line and an Ethernet line (which can't be tampered with because it's needed for an internet connection).

Simply running new cable from the closet to the garage is unfortunately impractical, since it would need to be underground (the garage and main house are separated from each other) and we can't do that.

skip to here to ignore background info ramble

The only solution I can come up with is somehow modulating the voice signal over the existing coax line and demodulating it at the other end (and vice versa because voice is two-way), which I have no idea how to do (or if it's even possible). The main problem here is that my current modulation setup is one-way, video–>coax–>TV, where the TV never interacts back with the modulator or video source. This is obviously incompatible with voice (and IP, etc), which means I would need some sort of modem on both ends rather than a modulator on one and a demodulator on the other. Regardless, if it is indeed possible, how could I go about running voice over coax?

Expanding on this I'm also curious if I can do "general purpose" modulation, wherein I could run stuff that's completely unrelated to what coax is usually used for, like USB or SPI (for Arduino), over the existing cable. I've ran USB over Cat5 cable before with success (the 4 USB wires connected to 4 of the 8 Cat5 wires on either end), which is what brings up this idea.