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Theoretically RF signals will not go thru ideal magnetic material. But in real practice nothing is ideal and part of RF energy may leak. There are a lot of different scenarios, but in general it is hard to get better than 90 dB shielding attenuation.
In case of handheld device where you have no fast moving parts, constant magnet should not create problem. In cell phones magnets widely used for having electrical signal of lead opening in clamshell form factor designs. It is probably good idea not to place such magnets close to microphone or speaker, some kind of displays. Also magnet may increase slightly microphonic effects on the board, but it should not be a serious problem.