Copper-coated Cat5 is popular in developing nations, and is susceptible to corrosion, but only where it's PVC coating has been removed, such as the end terminations. You may be able to snip off the ends, expose some bare wire and re-terminate them.

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If it is a CAT5 cable you could solve it by cutting the ends ... or simply change the socket as the contact might be broken.

If it is a phone line they might have used a spun cable (cheap) that has a plastic center and a spun thin coppar plated wire around it.

The best choice is to have a outside wheather resistant cable indoors to.

It could be that someone suggested that they change and then sell the old cable to someone else as valuable scrap

Copper does not rust only gets a greenish oxidized film on its surface.

But being where you are (TIT) your wires are possibly not copper, only copper plated. Could be steel or aluminium.

Than they corrode, much faster at the seaside where you have moisture, salt and iodine in the air.

Check your wires. If 'springy' - it's steel. If soft but break easily after a few bends - it's aluminium. Sorry, mate, TIT.

And forget the 'farang' song, the bloody bastards do this equally to locals.

If the condo is yours - you will have to cough it up. If not - you don't have to pay, just live without internet.

Rust IS oxidation. You may be confused and believe that iron oxidation is the only 'rusting'.

To the OP: Yes it is quite possible that the building's cabling is the cause - especially if your internet is jittery.

Chiangmai Riverside Condo has that exact problem. Who knows when they will fix it as everyone seems to be in denial. I'm a retired international computer consultant and have enough direct experience to know how to run the test for this and did get my ISP to come out with the proper equipment. As they expected their feed to the building is fine but the feed in my apt is not. When the building (its a condo) accepts that and when they will fix it are anyone's guess.

FYI: Water + salt + copper = oxidation at a very rapid rate. Removing the salt from the equation and copper holds up much better to water exposure than iron, steel or aluminum. If you live near an ocean they may have used salt water to mix the concrete. If so you need to have a plastic sheath around the copper or rain will destroy the cable quite quickly!