It could be the case too, but they should receive a pdf attachment specifying what is missing or incomplete. We initially got one email like that, stating which form was missing something (n/a missing in one line).
I doubt this is the case, though, as when we received an email about missing/incomplete documents (with the attachment to specify what was wrong), we didn't get an AOR at all (no UCI or App number). Pretty sure they wouldn't have added you in to the system if your application was missing crucial documents when received, but they would return your application, emailing you about it beforehand.
As you said and myself, bets to try calling. If no results, think of reaching out to a MP.
as the purpose of AOR1 is to confirm they have everything they need from you when they receive the application, this is a good point:
Pretty sure they wouldn't have added you in to the system if your application was missing crucial documents when received, but they would return your application, emailing you about it beforehand.