They treasure their life/lives?

May I ask which one is correct? "They treasure their life" / "They treasure their lives?" Everyone has only one life but since the subject is "they", I don't know if I should use the plural form or the singular form. Also eg. they put their cell phone on the table. Every one of them just has one cell phone but when I mean they all put their cell phones on the table, which one should I use? "They put their cell phone on the table?" or "they put their cell phones on the table?"
Thank you so much for your answer.

You can use either. (There seems to be a trend towards saying everything in the plural these days, but it’s generally less elegant, in my view.) Context is all-important, since there’s usually an element of ambiguity in this sort of expression, whether you use singular or plural:

I recommend lives. Different people don't really share one life. It doesn't make much sense to imply that they do.