What are the meaning modern telephone circuit? The transformer is for isolation which is safety issue and for some old ringer circuits. If you means featurephone, the marketing is already dominated by Taiwan source (Winbond, UMC ...)
Again, the signal transformer is used for isolation and signal coupling in DAA circuit. If your design haven't any exposed metal, you wouldn't need any transformer. You just tape the signal from so-called audio in. You can found some design in Panasonic telephone service manual in Manual Database section.