The Duo model also has the 3.5 mm jack, it's just not listed on Digitec. If you google both models, you always end up on the stereo model page at Jabra. I can't find the Duo model at Jabra.

I suspect it's an older version of the same product, or the product was renamed at some point with the same specification.

I asked myself the same question and came to no conclusion. I simply bought the Jabra Evolve 40 UC Stereo (cable) for my private use for PC, Mac and mobile and am extremely satisfied with its performance, comfort and price. Everyone in our company has it too.

Sorry, typing error. I meant the 3.5 mm jack, of course.

40 UC Stereo gives stereo reception. UC 40 Duo gives mono reception. Duo = headphones with two ear cups.