To unlock a secret Darkling go to any pay phone in the subway and dial 555-4263 (555-GAME). 
You will hear a message to confirm correct code entry.

Collectable 1:
Go to any pay phone and dial 555-4463. You will hear a message to confirm correct code entry.

Collectable 2:
Go to any pay phone and dial 555-4525. You will hear a message to confirm correct code entry.

Collectable 3:
Go to any pay phone and dial 555-4565. You will hear a message to confirm correct code entry.

Collectable 4:
Go to any pay phone and dial 555-4569. You will hear a message to confirm correct code entry.

Collectable 5:
Go to any pay phone and dial 555-4603. You will hear a message to confirm correct code entry.

Collectable 6:
Go to any pay phone and dial 555-4649. You will hear a message to confirm correct code entry.

Collectable 7:
Go to any pay phone and dial 555-4676. You will hear a message to confirm correct code entry.

Collectable 8:
Go to any pay phone and dial 555-4682. You will hear a message to confirm correct code entry.

Collectable 9:
Go to any pay phone and dial 555-4691. You will hear a message to confirm correct code entry.

Collectable 10:
Go to any pay phone and dial 555-6118. You will hear a message to confirm correct code entry.