How would I break the ice to a colleague that needs to be reminded to not curse while another colleague near them is on the phone? I'm sure it came out naturally, which I don't mind, but I'd like to just to remind her to be more careful when she does it.
Just tell them exactly like you wrote it in your question. I don't think most people would be offended by that especially since you have a valid reason for bringing it up that they will understand.
Honestly, this is 100% about professionalism. "Please stop cursing when I or someone else is on the phone with a client. It makes us all sound like idiots".