A call may be placed on hold by the calling user,at any time after the call has been answered or additionally as a service provider option:1)2)after alerting has commenced,or after the calling user has provided all of the informationnecessary for processing the call.A call may be placed on hold by the called user,at any time after the call has been answered and before call clearing has begun.The Call Hold service includes the Retrieve operation which re-establishescommunication on a B-channel between the served user and the held party.The stage 1 definitions for the HOLD supplementary service are given in Recommendation I.253.2.The stage 2 descriptionsare given in Recommendation Q.83.2.The stage 3 DSS 1 descriptionis given in Recommendation Q.953.2.This stage 3 description of Call Hold supplementary service uses the ISDN User Part protocol as defined in Recommendations Q.761-Q.764.