The SIGINT indicates a methodology to gather information directly without an intelligence asset. The encrypted communication has been one of the sources of classified intel, which has been actually quite critical during the war time. If you have decoded its communication from the operation center, you can identify a whereabout of the enemy and their next move.
This direct gathering has been progressed, whose advancement is still within a range of the SIGINT, but its name is quite irrelevant. If you pay an attention to the name, you are likely lost to understand a true figure of this methodology. The critical point is that there is a way to gather information directly from the target without any assetization, which has also advanced trough the time.
This notion started with an interception of the communication and there have been many devices invented for this purpose. The listening bug is one of the simplest means, which is often enough to capture an intel of the target. The interception of the telecommunication was progressed from there, but it was actually not difficult at all as it was connected through telephone exchange. The current network is its extension so that the wiretapping itself is actually not so difficult even now.
Having said that, the current information is delivered by digital, not analog, even for the voice data. Some of those are encrypted when sending, which is necessary to be decrypted even when intercepting them.
On the other hand, when the spy set up a bug in your phone with an old school technology, your conversation is easily listened from the outside. This can be achieved through a mechanical setup of the wiretapping or might be by the hidden application.
It also holds true to the e-mail and chat that your writing is easily stolen by just installing the application, which itself is not an easy task, but not impossible.
These are categorized as an advancement of the SIGINT and, as you can see, it covers a wider range of the intelligence job than the SIGINT sounds. That is why it is better to recognize those as a methodology to gather information from the target directly without assetization.
It is still true that some of the inter-government communication is conducted by way of a wired encryption, whose cracking technology is advanced for this purpose, but it cannot be just applied to it. There are numerous communications conducted daily through each information device, whose data can be stolen by the same type of the decryption technology.