Hello
Its depend from what actually happen in the network.
I try to explain.
In reality we have some phase with call setup.
1. h225 tcp establishment.
2. Alternative route lookup if primary fail and so on.
3. Signaling.
What actualy happen is unclear.
If 1. you can try decrease h225 setup time on the
gateway.
if 2. this is trade-off between setup time and guaranteed service.
if 3. its again depend which node introduce delay and why.