IS IT OK TO TALK TO YOUR EX IF YOU ALREADY TAKEN ?

SO LETS SAY YOU HAVE A BOYFRIEND OR GIRLFRIEND, WOULD IT BE OK IF THEY TALKED TO THEIR EX BOYFRIEND OR GIRLFRIEND ON THE PHONE OR HANG OUT WITH THEM. THE QUESTION IS WHAT CAN RESULT FROM THIS KIND OF INTERACTION...r u putting the current relationship at risk b/c old feeling could be rekindled , or r u just reconnecting w/an old friend...or what r u guys opinions...