The three letter acronym ISM is short for Industrial, Scientific and Medical and is actually many different radio frequency bands but they share a common feature, anyone or anything can use those spectrum spaces without any legal headaches — providing they are not knowingly interfering with someone else’s devices. The RF spectrum ranges from low frequencies in the kilohertz up to multiple gigahertz for some of the satellite communications and different countries allocate portions of their radio air space to various uses.
Most people’s home wifi routers will transmit and receive on 2.4 or 5 gigahertz, and your cellphone is a small sliver of frequency space between 698 & 894 megahertz. While the 2.4 gigahertz devices land in one of the ISM bands, and therefore any type of device can use it, those caught transmitting in the cell phone ranges could soon find they have more than just Bell or Telus to worry about.
When you are looking to add radio power to your project it is easy to find small electronic transmitters through various online retailers. Before hitting “buy now” double check the frequency range the devices translate at. Double check that band if valid in your country, most governments will publish their spectrum allocation similar to this one for Canada.