How does Ukraine keep intercepting Russian military communications?

From the performance we've seen so far COMINT and SIGINT don't seem to be high on the Russian Army training list.

Poor communications security. Grunts complaining is probably the only military activity the Russian army properly preforms, but even that is screwed up because of repercussions.

This is probably happening in several ways. But at the most basic level, some Russian troops have apparently been far too casual in their communications.
The Russians brought their own cellphones into Ukraine. When the Ukrainians figured this out, they cut off Russian phone numbers from the Ukrainian network — so the Russian phones stopped working.
Russian troops then began seizing cellphones from Ukrainian civilians, according to Ukraine's State Service of Special Communications and Information Protection.
"We call on Ukrainians whose mobile phones were taken away by representatives of enemy troops to inform the operator as soon as possible," the Ukrainian agency said in a statement last month.