Modem - Guess it's for line coding, to use AMI, HDB3 or something like that. In that case it's not just a static signal, it depends on data to be sent.
You can do it with TTL/CMOS logic that drives the primary of the line simetrization transformer using 2 transistors - the transformer should have a central tap.
Basicaly, for AMI you take the data signal and AND it with the clock. Depending on the previous "1", the pulse obtained will activate one of the transistors that drive the trasformer, to produce alternative invesion of the pulse.
There are also a few chips for this:
**broken link removed**
Don't know if these chips are easy to find.