Quite an open ended question with no assisting information.
Mention if the issue is effecting one cell, BSC ,MSC.
if cluster based check for blocking it may be initiating the queuing.
if MSC based check the links they might be congested. if overall network check HLR / VLR links run traces at MSC to check at which phase of the call setup interogation the delay is coming.
Still some more information may be helpful