Set or get custom string associated with current JavaScript session. There are two kinds of the customData values: one is for JavaScript session (i.e., VoxEngine object), another is for the particular call (i.e., Call.customData and web SDK parameter of the method). It is possible to use them at the same time because they are independent entities. Remember that if you receive some value from web SDK, it does not overwrite the VoxEngine's value. Any of customData's type values can be later obtained from call history via management API or control panel.