If you don’t want to build or connect anything, just connect a Phone Headset and wear it during the recording session.
Your voice will be heard perfectly and you can hear the producer.
OTOH,
If you have a “pro” mixer with 600 ohm balanced LINE-LEVEL output(s) (2 conductors and a shield) you can simply connect one of those outputs to the phone line and the producer on the other end will hear your mixer’s output perfectly.
Build a cable that will fit the mixer’s output on one end, and the phone line (as follows) on the other.
1. Phone line RED: connect one conductor of the 600 line feed.
2. Phone Line GREEN: connect other conductor of the 600 line feed.
3. Leave the 600 line feed SHIELD unconnected.
First, make the phone-call (or answer the incoming call).
Then, connect your new cable.
Test and adjust the level so its comfortable to the producer on the phone.
Begin the session.
[You should still wear a phone headset (with its mic muted) and hear your audio send (pretty loud) and the producer talking (less loud) during the session.]
When finished, unplug the audio patch cable from the phone line, say goodbye to the producer and hang up.
If you know anything about electronics and parts, it would be slightly better to DC-isolate your board from the phone-line by connecting an electrolytic capacitor in series with the cable and the red phone line.
Any small polarized electrolytic will do. Here’s one at Radio Shack:
https://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=272-1017
220mfd 35v $1.29 (You can choose another small ELECTROLYTIC capacitor if RS is out of this one, it doesn’t really matter.)