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A buddy and I both bought a 20S after ignoring other riding buddies with SMH10s. Unfortunately, I picked the one that constantly dropped the intercom which was extremely frustrating and distracting. A call to Sena tech support and they recommended I sent mine back (after a reset, updating the updated firmware, etc) then wait 3-4 weeks for a new unit. I ended up returning it to Amazon instead. As it turns out, units with serial numbers under 140xxx had something wrong that couldn't be fixed and dropping the intercom was only one symptom. My buddy's Sn was in the 150xxx range and had zero issues.
I ended up going to Cyclegear and buying a unit with a serial number in the 150xxx range and all is well. The unit works as it should.
Moral of my story: buy one with the most recent Sn you can find.