This is the TORRII Knotty: an adjustable iPhone strap that lets you carry your phone around with peace of mind. You can use it to wear your phone around your shoulder or neck. Knotty is available in 6mm and 8mm versions and various colors.
This phone strap has a detachable design. You can remove the buckle on/off in a snap. You can use Knotty with a wide variety of cases.