Qualcuno è sulla porta.

… alla porta, forse?

Ciao Robertbehere - ho pensato lo stesso. Unless they’re talking about club bouncers or theatre welcomers;-)

English Translation

Someone is at the door.

No, I think “sulla” is also correct. Whereas in English “door” is primarily the “door leaf”, the iItalian idea of “porta” is foremost the place of the door frame or in former times (Latin) the archway.
If I got it right "sulla porta is more like “standing in the (open) doorframe” or “on the door step”
“Alla porta” is more like “in front of the (closed) door”.