When to speak: never.
What to say if you do speak:
Am I being detained?
If you're being detained:
Are you charging me with a crime?
If they don't let you leave after that:
I am exercising my fifth amendment right not to incriminate myself and my 6th amendment right to counsel.

Any other question, continue to repeat your assertion of your right to counsel.

Never answer any question, just ask them the above questions. And if they try to search you or your vehicle, say very clearly "I do not consent to any search."

And I think it's prudent to have a lawyers phone number memorized or written on your skin in permanent marker. Or one of the toll free hotlines listed by other posters here.

Don't unlock your phone while in police custody. And before the action, disable all biometric phone unlocks in favor of a strong password.