The cell phone itself will not interfere with wireless at least as long as it passes government regulations. You can see the test results they are required to file with the FCC if you really want to dig. The frequencies the cell phone operates on is different than WiFi and they are required to prove they do not transmit above certain levels outside the frequencies used for the cell communication. Because the bands they operate on are licensed it is very important they do not tranmit outside these ranges. Not because they interfere with wifi but because they interfere with other cell phone providers who license adjoining frequencies as well as things like military frequencies.
Now if what you mean is if you turn the hotspot...