Most PBXs can associate multiple calls with a single line. A call can be placed on hard hold, which frees the user's line/address to make other calls. An application can place a call on hard hold by calling lineHold. An application can retrieve a call on hard hold by calling lineUnhold.
Hard hold is different from a consultation hold. A call is automatically placed on consultation hold, for example, when a call is prepared for transfer or conference.