The functionality described in the documents linked to below is for OTG-enabled devices only.
If you are implementing code for a Symbian platform phone that will support USB host/On-The-Go (OTG) services, you need to have a thorough understanding of the On-The-Go Supplement to the USB 2.0 Specification.
On a Symbian platform phone that supports USB host/OTG services, the USB control application needs to monitor:
Requests from a local application for the phone to become the USB host
Requests from a remotely connected device for the phone to become a USB peripheral
VBus state
ID-pin state
OTG state
USB bus activity
The documents linked to from this page tell you how to monitor the USB Manager for these things, how to control when the phone is and is not the USB host, and how to drop VBus when that is necessary.
The final document linked to below tells you how to make requests from USB-aware applications to the USB Manager for the phone to become the USB host. When an application makes one of these requests, the USB Manager notifies the USB control application, which responds by issuing a request to the USB Manager to attempt to make the phone become the USB host. (A USB-aware application cannot make this request directly, only the USB control application can; see Locking applications out of the USB Manager.)