*** Spoiler - click to reveal ***This is early on in part 2, just head to the diner and explore the alley to the side...
After getting knocked out, Ray wakes up in the Sewers...
Head to the left and exit the room, keep heading left past the gate.
Check the graffiti out and open the phone box, you need a
dime to make a call, make a note of the
name (I think it's random).
Switch to Reyes and head to the bank, you should see a
dime on the floor. Pick it up and head to the Quickie Pal and drop the
dime into the Sewer manhole cover.
After the short cut scene (still as Reyes) head on over to the phone box on B street. Look at the phone book and then look up the
name from earlier. Make a note of the
phone number.
Switch back to Ray and head back to where you woke up and go through the north exit. Grab the
dime on the floor and head back to the phone box and graffiti.
Use the
dime and dial the
phone number, speak to some random guy. Grab the
dime from the coin slot and exit right (back to the gate).
The Sheriff-a-reno should open the gate and set you free.