Daniel,
the blocking on VoIP is the almost the same in Tradditional telephony. The resources are similar:
For example, if you have a PBX with 30 Trunk (Lines), the you have only 30 resusrces tha can be used for calls
In VoIP those resources are the Codecs that you will need to convert the analog to digital Voices. whe you try to acomplish a call num,ber 31, you will have a blocke call (either in PBS or in the gateway)