Yes telephone exchange line fuse, there to protect against high voltage applied to the line, think a power cable drop on it, and then the fuse blows, lighting a LED on the row to indicate a fault. This one came from my stint as overnight phone operator, where there was one faulty line, which was annoying, in that it kept on cycling the old mechanical relay PBX, it only had pulse dialing, none of this new fangled DTMF around, and in any case your upstream connection was via an operator connected call, and pulse dialing was only used internally. Thus after 4 hours of listening to the Cerchunk......cerlunk, of the exchange behind me cycling, and the corresponding lamp flashing on and off, I went and pulled this fuse, wrote the fault in the log, and had a much quieter night. 10PM town operator called in, they were going to sleep, and thus dropped me all the town lines to handle, as it was likely the calls would all be inbound to me. Farm lines went to the one remote operator 120km away as well, which was fine, as I did not have a ring generator there to use them, or the list of cadences per line for each farm. Morning I put the fuse back, and found the broken one dropped in a corner, so kept it.
Not the greatest 18 hour duty around, and with only typically 3 calls per night out, one from the Food Factory to place the fax call to send the order they had, and the others being calls from the Officers mess, to what I assumed were girlfriends, as the calls were around an hour each. Just write down the officer, time and length, and leave the day operator to send the chit for payment through.