13.6.4 Alternate from one call to the other
23.2053GPPBearer-independent circuit-switched core networkRelease 17Stage 2TS
When the hold request for the active call is immediately followed by a retrieve request for the held call the MSC server shall request the MGW to connect the bearer termination of the served mobile subscriber to the bearer termination of the held party. The MSC server also requests the MGW to both-way through-connect the bearer for the previously held call.