Just from the telephone short-number, you could determine whether the caller was an internal member, as well as their identity.
"Ah, yes! 711121!"
710, 711, 712, are prefixes for the Provincial Brigade, with 710 being for government agencies. If the number following it is 710001, then it's either the Regimental commander or Political Commissar, in short, a secretary, a member of the brigade's party committee—they're all within the "71000X" range.
711 signifies the large teams and Special Duty within the city area.
712 is for the county teams.
The prefix 711 for Cao Yi signals that he belonged to the larger squadron.
The suffix 121, the first "1" represents the Special Duty Squad number, the second "2" indicates the Second Squadron number, and the third "1" signifies Cao Yi's number—as the squadron's secretary, his number is 1.
If it was Ye Jiahong, his number would be 711122; Captain Zhang would have 711123, and Wu Di would have 711124.