If you call after 12pm and still get "not available" message. Then one of the following can be a possibility
1) You are green to the agent and the girl has enough customers for today.
2) You are blacklisted by the agent.
3) The agent is busy with something else at the moment.
Anyway, that's still better they don't take your call now than not taking a call after you made an appointment and made it to the hotel...