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.

Ukrainian civilians have complied, and this let the Ukrainian government know which phones were stolen by the Russians — and effectively became listening devices for the Ukrainians.