In the US. As I do more irl stuff, I am becoming concerned about being arrested at a protest or whatever. I've not yet come anywhere close to this happening, but should I do some prep like this ahead of time? Are there PSL approved lawyers or something, how should I know who to call? Will I need to have the number memorized? Can I cold call them post arrest or should I form a relationship first? I don't want to get billed.
I wouldn't bother. IMHO you're better off just learning a bit about what to do and not to do during an arrest (e.g. when to speak and what to say, if anything). No lawyer is going to be helping you with that in real time when you need that information.
Once arrested in the US you have the right to be provided an attorney at no cost. If you get one phone call I would not be wasting it on some lawyers guild. Call your family, friends, or whoever you need to get word out to for financial help or coverage while you're in jail.
Public defenders get shit on for many reasons, and they're overworked and underpaid, but they can still be helpful to you for what you're describing.
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.