A ringing detection, and audio coupling circuit can be done easily:
When a ring comes, the AC signal 60V 20Hz feeds the opto-coupler LED and
the -RING signal goes to ground, the microcontroller senses it and
drives the relay that closes and connects the 600 Ohms phone transformer
to the line.  For most applications, the ringing sensing circuit
(capacitor, zeners, ...) does not interfere so much in the common phone
line usage for voice or data, if still connected in parallel.  If you
want, you can use a SPDT relay and switch the phone line *from* the
ringing sensing circuit to the phone transformer, but it is not strictly
necessary.  Of course there are circuits that can do all of the below
easily, but as usual, at cost of more dollars.

phone line
o---------o--------------------o    o-------------.    
          |                    ------             |     .-----> 
         ---  0.22µFx250V      relay              |     |
         ---  0.47µFx250V                         'S | S'
          |                             600 Ohms   S | S  600 Ohms
          |                                        S | S
          V  Zener 22V                            .S | S.
         ---                                      |     |
          |                                       |     '----->
         ---                        o +5V         |
          A  Zener 22V              |             |
          |                         |             | 
          |                         R 10 kOhms    |
 2 kOhms  R                         R             |
          R                         |             |
          |         opto coupler    |             |
          o-------.   4N35  .-------o---->        |
          |       |         |          -RING      |
         ---      V __--> |/                      | 
  1N4007  A      ---      |\e-.                   |
          |       |           |                   |
          o-------'           |                   |
          |                  GND                  |
          |                                       |
o---------o---------------------------------------'
phone line

German wrote:
> 
> I wonder if there's a one chip (or simple) solution to detect
> ringing a go off hook. My PIC circuit understands DTMF and call
> progress but I'm a bit lost with the telephone interface.
> Any experience out there?
> 
> Thanks!