There are a lot of Android devices, Android versions, and Bluetooth devices from different manufacturers. This leads to a lot of combinations. Because of the variation in handset/device combinations, it’s very difficult to say why it happens with any particular set. It could be limited resources, an incompatibility in Bluetooth versions, one of the devices in the wrong mode, or one of many other reasons. In the Vivox SPAM logic, we retry up to three times with a 300 ms delay between each attempt in order to maximize the opportunity for a Bluetooth SCO device to connect correctly.