Well why don't they do a proper test and send up a CRJ with cell phones on only in row 9. They can repeatedly switch the phone on and off and see if the problem is consistent.
It's worth a try.
What's not a good idea is dismissing the possibility, outright.
Interference can be tricky to reproduce, sometimes. For example, the shielded wiring may have its shield grounding point(s) going intermittent. On the ground, all is fine; once in the air, with the wing bending up a bit, the wiring shifts and not so RF-proof anymore.
Once in the signal wires, RF can "mutate" through a variety of mechanisms down to the interference frequency. Even somewhat oxidized connectors are known to make a poor rectifier. Show me a schematic and we can start hunting for potential mixing hazards.
Anyhow, the sporadic nature of the reports of this sort, that I've come through over the years, makes me think that the problem could involve aircraft maintenance state/assembly quality, not just the design.