A "jammer" means a device that broadcasts EM waves, to drown out any legitimate signals. Such a device would be a massive FCC violation if ever turned on. Go straight to Federal prison, do not pass go, do not collect $200.
If you mean something like a Faraday bag, that may work. Any Faraday cage results in a net electric field inside of it of zero - so the phone would not be able to detect the cell network. It would still be pinging like crazy and that could escape the Faraday cage... unless you ground it.
Faraday cages also have to be designed to block frequencies. For cell phones, you would have to block multiple freqs at once.
A discussion on it. https://physics.stackexchange.com/questions/149607/what-is-the-relationship-between-faraday-cage-mesh-size-and-attenuation-of-cell
Something like this for personal use, one would just use aluminum foil. Being solid (non-mesh) it should work for all frequencies a cell phone would use.
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