Robocalls have become unavoidable, flooding phone networks with spam so thoroughly that most people have stopped picking up calls from unknown numbers. It’s a massive and embarrassing technological failure, but the causes behind the problem are more complex than you might think.

The biggest problems are baked into the network itself. As the internet moves to fiber optic cable (the modern broadband network), phones have, too, sharing infrastructure with the public internet and cable TV. That’s come with new risks, as the phone network grows more exposed to internet-based attacks. But it also has new rewards, as the IP-based system can execute more complex authentication protocols to ward off spam.

The problem is, US networks are only partway through the transition, which leaves us with the worst of both worlds. Much of the US is still waiting for broadband and still running its phone system on copper wire. We need those phones to keep working, which means the whole network has to be backwards-compatible with the old copper-based system.

If that weren’t true, the robocall problem would be much easier to solve. There are already protocols that would protect the phone system against robocalls, but those systems are designed to run on the new generation of IP phones, and they have trouble telling the difference between a spam caller and a copper wire phone.

The Federal Communications Commission cleared the way for more aggressive action by carriers earlier this year. But it’s a sticky problem, and the only answer may be laying more fiber. As long as parts of the network are lagging behind, robocalls could be the price we all pay.