The (Motorola MT500) Radio is the main way of communication between the Ghostbusters team when separated. It was most famously used in the scene at the Sedgewick Hotel where Slimer slimes Peter Venkman, while Ray Stantz tells him that it wont hurt him.
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In the first movie as he starts yelling as Slimer attacks
The Radio Walkie Talkie as seen in Ghostbusters The Video Game Realistic Versions