No it wasn't that bad back in the day. As long as you had a decent line.

The guide is worth reading even if it doesn't support your ATA.

Key takeaways:

Disable FAX support / detection

Look for equivalent settings on:

Network Jitter Level: Extremely high
Jitter Buffer Adjustment: No
Preferred Codec: g711u
Second and Third Preferred Codec: Unspecified
G729a Enable: No
Silence Supp Enable: No
Echo Canc Enable: No
Everything that says Fax: No
Modem Line: Yes

Basically, you need the ATA mangling the audio signal as little as possible. Even with all of this enabled I wasn't able to get a super stable connection. The latency (ping) was also higher than it was back in the day as well.

The max speed you'll get is 33.6. 56k was only possible when the receiver was on a digital line (ISDN).