This is an old article, but what the heck: DVD Savant on Paul Frees in Some Like it Hot. The fact that Frees dubbed Tony Curtis's "Josephine" voice in that movie is now fairly widely known, but apparently it was not well known a few years ago, and I don't think I ever heard either Curtis or Billy Wilder refer to this in interviews.
As Savant notes, Frees became the go-to guy for overdubbing in the '50s and '60s, when dubbing, or "re-voicing," suddenly became more popular in American movies. Up until the late '50s, American movies usually mostly used direct sound, and therefore the voices on the soundtrack were the voices of the people onscreen, recorded simultaneously with the visuals. Dubbing was mostly used in musical numbers (which don't use direct sound) for actors who couldn't sing.
In the late '50s, however, while American movies continued mostly to use direct sound -- unlike, say, the Italians, who preferred to shoot the entire movie without sound and dub everything in later -- the use of post-recorded voices became more and more prevalent. I suspect the main factor in this was the increased use of location shooting. If you're shooting in the studio or on the backlot, you can always assemble enough actors who can speak their own dialogue. If you go on location, and in particular to foreign locations, then it's harder to record the dialogue (and you can ill-afford to stop the take and start again if someone flubs a line in an exotic location), and it's harder to find actors who can speak English to play the minor parts.
So what happens in American movies from the late '50s and '60s is that you'll usually get some interior scenes with direct sound, especially scenes between the principals, but anything shot outside will have post-dubbed dialogue, and anything shot on a foreign location will have the minor characters post-dubbed by an American actor. And that actor was usually Paul Frees. In Gigi, which was mostly shot on location in Paris, there are some scenes where Frees voices every single character except the principals; he did similar duty in Spartacus. And sometimes, as in Some Like it Hot, he would be called in to dub a well-known actor who couldn't quite get the voice the director wanted. Someone should compile a list of all the uncredited voices he did in live-action movies; there must be hundreds.
Speaking of Billy Wilder movies with post-dubbed voices, my favorite example of this is in One, Two, Three -- a movie mostly shot indoors with direct sound, but with one character entirely re-dubbed. This is the Count von Droste Schattenburg, a down-on-his-luck German count, who is played by Hubert von Meyerinck, but whose voice is entirely dubbed by the great character actor Sig Ruman. My guess would be that Wilder wrote the part for Ruman, and Ruman wasn't available, so Wilder wanted to at least use Ruman's unique voice for the role.
Incidentally, the question of direct sound vs. post-dubbed sound was quite a vexed question in film-geek circles in the late '50s and early '60s. Francois Truffaut made his first three films (The 400 Blows, Shoot the Piano Player, Jules and Jim) with mostly post-dubbed sound, mostly for reasons of budget and convenience, and other young French filmmakers like Alain Resnais were also making movies with post-dubbed sound. Jean Renoir, the grand old man of French filmmaking, said in 1959 or so that he liked movies like 400 Blows and Hiroshima Mon Amour but chided the young filmmakers for, as Truffaut recalled it, "Not defending direct sound." Renoir felt that post-synchronization was a poor substitute for having the dialogue recorded simultaneously with the rest of the performance. French filmmaking eventually shifted back toward direct sound (Truffaut always used direct sound after Jules and Jim, and in Day For Night there's a joke about an Italian actress who isn't used to learning her lines, and has to adjust to the French preference for directly-recorded dialogue); the Italians mostly stuck with post-dubbing, as you know if you've ever watched a Sergio Leone Western.