The gateway reserves it at step four of a call, after the mandate has allowed the call and the input has been checked, and before the agent is contacted. The same amount is reserved against the mandate's budget. Parallel calls see each other's holds, so ten agents working at once cannot together overspend a budget that none of them exceeds alone.
A hold is temporary and is not a movement of money, so it is not written in the journal. It exists only while the call runs. When the call settles, the real cost is charged and the rest is released. When the call fails, times out, crashes, or the gateway itself errors, the whole hold is released and nothing is billed. A restart releases the holds of the calls it interrupted.
Example: an agent listed at up to 3.00 EUR is called with a max cost of 0.20 EUR. 0.20 EUR is held, the run costs 0.06 EUR, and 0.14 EUR goes back.