The icon is a logo for the Callback Feature. The icon is what appears to be the shape of an old phone. The phone is at an angle with the top of it to the top left. There is a curved arrow that starts at the mouthpiece and points to the top.
This is a Callback icon. We've made it in iOS style
, first introduced in iOS version 7 and supported in all later releases up until now (at least iOS 11). This style is based on thin two-pixel lines and is optimized for 50x50 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 13,300 icons, all in the same style and quality.