I always find that whenever Pheobe says something funny and just before the camera changes angle, Lisa Kudrow makes a face that is similar to someone holding their laughter. I have seen the gag reel and there are a lot of scenes where she was laughing because of the lines. But there are some instances where she says something funny and makes that "holding laughter back" face. What I try to figure out, is that PHOEBE that makes that face, or does Lisa Kudrow? Any interviews or articles that describe this?
Friends is a sitcom, and a lot of sitcoms and other comedies1 are kind of meta, in that the characters will say something funny for the audience's benefit but the actors will laugh or almost laugh. The filmmakers often leave this in the show because, even though they technically broke out of character, the show isn't meant to be all that serious in the first place and audences might be more likely to laugh if they see the actors laughing or almost laughing.
1 For example, in the comedy sketch show, Studio C, the actors often visibly struggle to repress laughter and keep a straight face, but they leave that in the show because often it makes it even funnier. :)
Friends, as a show, rarely explicitly breaks the fourth wall. It's more accurate to say that all of the characters on the show frequently, implicitly, lean on the fourth wall for humor (Chandler about babies names, Rachel referencing fan obsession with her hairstyle at the time, etc..). There were a handful of one off exceptions, eg. Ross in a laurel and hardy homage.
Phoebe's infrequent 4th wall breaks stand out because they are performed in character, as part of regular episodes.
So while I don't find any specific references to her "holding laughter back" face being intentionally allowed screen time more often than for other characters, it would make sense as a decision.