Nothing actually has changed; the regular phone network has been digital TDM for ages. If you disabled the echo cancelling and use an uncompressed codec like ul

Nothing actually has changed; the regular phone network has been digital TDM for ages. If you disabled the echo cancelling and use an uncompressed codec like ulaw it's always worked to run a modem over an RTP stream, provided you were able to guarantee zero packet loss and zero frame delay (no jitter). But if you are asking if this will really be reliable over the public Internet, the answer is still pretty much no, not well. Any network hiccups or delays will often just result in the connection dropping completely. The advice to steer clear of such solutions is still as valid as it ever was, but for a hobby retrocomputing project, yeah knock yourself out.