22.0013GPPPrinciples of circuit telecommunication services supported by a Public Land Mobile Network (PLMN)Release 17TS
This condition occurs, when a call is about to be offered, if the information (i.e. traffic) channel is busy and the maximum number of total calls has been reached (see note).
This condition also occurs, when a call is about to be offered and an already on-going call attempt (incoming or outgoing) is in the establishing phase, i.e. not yet active.
When NDUB condition occurs, the PLMN will clear the call and indicate "busy" back towards the calling subscriber (see also clause 4).
NOTE: The value of the maximum number of calls is 1 for the basic call. When the supplementary service "Call Waiting" is applicable the value is n+1 where n is the maximum number of calls that can be waiting.
3GPP TS 22.135  defines NDUB for Multicall environment.