1: Harmonics and intermodulation products
Harmonics are a normal product of almost all RF transmitters. It is common to see this effect when you have multiple transmitters sharing antennas or feeders, or in cases where contaminated, corroded or bad surfaces cause passive intermodulation (PIM).
2: Equipment transmitting in not allowed bands.
On occasion, service providers may have equipment transmitting on unauthorized bands. For example, radio stations or microwave links.
3: Another common case is interference at the borders between countries.
Spurious emissions from electronic devices. Almost all electronic devices emit RF energy at different frequencies. When these signals, or their harmonics, are of sufficient amplitude and fall within the cellular band, they cause interference. Examples: Amplifiers, lamps, screens (neon), industrial equipment.
4: Other region electronic devices
Devices that operate in bands allowed in one region that interfere with bands in others. Examples: Radios, microphones, cordless phones.
5: Repeaters with improperly adjusted gain.
Private entities can install repeater antennas to improve coverage in a certain band. If these items are set with excessive gain, they may cause interference.
6: Intentional Interference.
It is common for some private individuals or government entities to use jammer devices to limit mobile communications in an area.