Phoebe Buffay's hair changed enough across Friends that a single yes-or-no answer is too simple. Some looks appear to rely on extensions, styling pieces, or wig-like continuity work.
Why sitcoms use extra hair
Long-running shows need continuity. If an actor cuts, colors, or damages hair between seasons, the hair department may use extensions or pieces to keep the character familiar.
What viewers notice
Phoebe's hair sometimes looks fuller, longer, or more uniform than natural day-to-day hair. That does not automatically prove a full wig; it may be extensions or styled pieces.
Bottom line
Lisa Kudrow's Friends hair is best described as TV hair: natural hair plus professional styling, and likely extra hair in some looks rather than one permanent wig answer.