In static war field telephone links were suspectible for being intercepted. Finns used Swedish in some cases, as educated Finns in addition to Swedish-speakers could speak Swedish, but finnish speakers employed by the Soviets usually could speak just finnish. Or that was at least the theory.
That was the theory, and generally it was probably true. Many of the men from Finland who were officers in the Tsarist army came from Swedish-speaking families, and some of them joined the Reds in Russia, too. Men like later Admiral Axel Berg, say. But they definitely were a small minority. There were also some Finland-Swedish Reds who escaped/moved to Russia in 1918 and after it, but the numbers would have been absolutely and comparatively very small. One example would be
Eyolf Mattson. Of these groups, and of the well-educated leading Reds of 1918, who at least spoke some level of Swedish, not many would have been available during WWII to listen on tapped field telephones. They had either been purged, or then they were engaged in very different sort of work.
By the way, do you know if someone has researched the fates of Finland-Swedish Reds/Communists in the USSR, specifically, not as part of a general study about Finnish exiles in the Soviet Union?
As for the question in the OP, maybe Finland could just use people from Rauma? Nobody understands what the hell they are saying most of the time, and in the WWII timeframe
the dialect would still be spoken more than today.