How do you diplomatically tell a coworker not to call you at night to talk about work?

Just don't answer your phone

That would never happen to me, since my smart phone is always on Do Not Disturb mode, and I turned off the ringer on my landline phone, so all callers must leave messages.

I could not be diplomatic sorry ...

I would say I made a conscious decision to separate work from my home time and I stick to it. Please do not call me to talk about work. The time to do that is at work. My home time is precious. Thank you. ( Some people cannot draw lines. It can lead to heart attacks, strokes, hypertension, depression and much more). If that co-worker cannot hear you, block his number or let voicemail take it. I am glad you are making a line concerning this.

Tell them you can talk about it as soon as works starts.

don't answer the phone

"We're not at work, so shut the fuck up about it."

Say you need some downtime. Nothing wrong with that

'please don't call me at night to talk about work'

When he calls, don't answer his question. Instead just say "Let's talk about that tomorrow at work!"

Hes texting? Dont reply. I put my phone on silent every night

Was it for emergency?

Caller ID, not pick up?

Tell them your business hrs are from 9-5. After that it's "me time"!

Off the clock we don't talk about work.

Tell them exactly that.