Perhaps they recently got a new PBX (or this was a new hotel, the PBX was installed recently) and when they did the wiring they got the wire going to your room reversed with the wire going to another room. When the guest from the other room made calls, it appeared as if the call was coming from your room instead.

When my office recently got a new phone system, this is exactly what happened when the guy who did the wiring did a poor job. These two people in adjacent offices kept getting calls that were meant for the other person.

On the other hand, the hotel might be using an IP phone system where each extension/room number is associated with the MAC address on each IP phone. What might have happened was that maybe a phone wasn't working properly in another room and somehow somebody from either housekeeping or engineering who had no knowledge of how IP phone systems just took a phone from an unoccupied adjacent room (which could have been yours) and plugged it into that room. They then failed to notify the phone system administrator that the switch occurred so that the MAC address of the IP phone can be reassociated with the new extension/room number.

Because that phone's MAC address is associated with your extension/room and is in another room, when the guest from that other room made calls, it appeared on your bill as if it came from your extension/room instead.

It could also be possible that a guest in another room is savvy enough to "hack" into the IP phone and change it's internal configuration so that it would register to the IP phone system as another room number. (Whoever setup the system in the first place did not do a good job and kept all the passwords as the phone manufacturers's default).

I have been finding that a lot of the newer hotels tend to now use IP based phone systems.