Whenever I'm on a phone call on Discord, the sound quality drops significantly. I have the high quality streaming option switched on, so the call must be interfering with spotify somehow. I have a Macbook and premium account.
Are you using headphones and/or a headset? I'm pretty sure this is Mac OS or Discord switching that to call mode, which activates the microphone and results in reduced audio quality. The next time this happens, go in to System Preferences -> Sound -> Input, and make sure the input device is set to Internal Microphone (or something similarly named, it should have Type Built-in).