Most PBXs can associate multiple calls with a single line. A call can be placed on hard hold. This frees up the user's line/address to make other calls. tapi32.dll can place a call on hard hold by invoking TSPI_lineHold. tapi32.dll can retrieve a call on hold by invoking TSPI_lineUnhold.
Hard hold is different from a consultation hold. A call is automatically placed on consultation hold, for example, when it is prepared for transfer or conference.