Did the manager actually go through the call logs (or was she just assuming that that's how those calls were connected to your room)?
If it's the latter, then it's possible that the caller simply used a random room number, and connected via the hotel's PBX system (where your room number = an extension number), or DID (some hotels have direct numbers, allocated to each guest room).
I had the same thought -- how would the manager know that the caller asked for the OP "specifically by name and room number"? The OP's conversation with the manager came long after the calls from the prostitutes. The hotel might have a record of an incoming call to a particular room, but I doubt very much that a hotel worker would make a record of how the caller specifically asked for the OP.
That leaves the possibilty that the hotel workers on duty remembered that the callers asked for the OP by name. Here again, why would they remember something so trivial? This is a big hotel, right?
My guess is either 1) what KVS describes, above, or 2) there's someone at the front desk guessing that certain men might like some company, so they have their lady friends call the room. Annoying? Very. A security issue? I doubt it.