Background noise during calls on the Xiaomi 15 can occur when the device is used in areas with extremely weak signal reception. In such conditions, the phone can struggle to maintain a stable connection, leading to static, distortion, or unwanted background noise. This issue is not specific to the phone but can happen with any smartphone under similar network conditions.
To improve call quality, try making calls in an open space or moving to a location with stronger signal reception. Additionally, enabling Wi-Fi calling (if supported by your carrier) can help reduce noise in areas with poor cellular coverage.