Recently on a trip to Orlando on Sun Country, I came across a cabin chime that I'd not heard before. It sounded like two 'high-low chimes' in a row, so basically equivalent to someone ringing a doorbell twice.
I know that one 'high-low' means that the FAs are speaking to each other through the phone in the aircraft, but I've never heard it twice in rapid succession.
Are all 737-800s equipped with these chimes? What do they mean?
As far as I know, the chimes work the same on all 737 NG (and Classic) versions, although as was mentioned in comments, how they are employed will vary from airline to airline.
The two tone "high-low" chime described in the OP is the "crew call" system. The pilots have a button, the forward F/A station has one, and the aft F/A station has one. Regardless of which button is pushed, the "hi-low" chime is the same. In many, though not all, cases, this chime is a signal to the F/A's to answer the interphone. If the pilots press their button, they're asking to speak to the F/A's; if one F/A station presses the button, they're asking to speak to the other F/A station. (They can also press a different button that calls the cockpit, but passengers won't hear that chime. In the cockpit, we hear that "call" as the two-tone chime; we don't hear the chimes in the cabin unless we're listening to PA audio.)
At one carrier, a single high-low chime may mean "answer the interphone," while at another carrier, one chime might be used to mean "secure the cabin" while two chimes is "answer the interphone," three chimes is "answer the interphone - urgent," and so on.
You'll also hear chimes when the F/A Call Button above a passenger row is pressed; this is a distinct chime (a single, relatively high note). There is also a chime when the Fasten Seatbelt sign is turned on or off, this is again distinctive, a single lower note.
Note that you won't necessarily hear the chimes when the interphone is in use; it's simply a "Hey!" call to the Flight Attendants in the cabin. While it's often followed by a conversation on the interphone, they two are separate systems and one can be used without the other.
Ralf has said it in a great way.
The chimes are part of the cabin / crew intercom. What they mean depends. The carrier I work for, uses a single chime to advise the cabin crew that they have to secure the cabin. Typical calls are indicated by the two high-low chimes you have described. Whether you hear them or not depends. In many cases - especially in smaller aircraft - using the chime to call is not required, since the cabin is relatively small and the crew is close to each other.
In this case a chime can only be heard, when the cockpit calls the cabin. In exchange we can hear it, when the cabin calls us in the cockpit. Other than that, pilots do not hear what is happening in the cabin, unless they link themselves into the intercom of the cabin crew, which is - at least by us - rarely done, as there is no reason.
Cheers
