To feed or extract audio to/from a telephone line, from any chip / source / input / output with a transformer, connect any audio output-type transformer secondary (low or 8Ω impedance) in series to one telco wire.
Connect the primary leads (or high impedance) to the chip you mention output pin and ground.
Telco-------------------------------------Telephone
Telco---------------uuu-------------------Telephone
..........................=====
DTMF out pin------nnnnn------gnd
(ignore the dots)
To use a 1:1 ratio 'wet' transformer, it better be a ~600:600 impedance and can be connected in parallel to the telco wires.
Telco----------------./ .----------------W600Ω 600Ω---< audio pin
Telco-----------------------------------W600Ω 600Ω---< gnd
W= 'wet' winding end
Edited: added---> the DTMF level injected into the telco line should be around 0.3V rms. (~1V p-p)
Miguel