brandifolk, calling or emailing coopculture prior to the dates/times tickets go on sale will not get you tickets any faster. You need to go by the on-sale dates/times listed on the website. Yes, they sell out VERY quickly.
Another trick is just to try and book the tickets that are less in demand, such as those for "Audioguide of the Colosseum with ticket Colosseum, Roman Forum, Palatine, Imperial Forumh24" or "Audiovideoguide of the Colosseum with ticket Colosseum, Roman Forum, Palatineh24". You won't get into the underground but the fight for THAT ticket isn't one I'd even have the stomach for. Either of the two I've linked will at least get you inside, and you can see down into the underground from the points above that are not covered by the partial arena floor.
You still have to wait to book until you see that these tickets are available though: 30 days or so in advance, and they will still sell out very quickly. This is the #1 tourist attraction in Italy, and you'll be competing with thousands of others to land tickets to get inside.
Recommended companies for tours/tickets independent of Coopculture's are Walks of Italy, The Roman Guy, and Through Eternity. They will cost you more than Coopculture tickets, and I wouldn't book their tours which include the underground as there's a chance they might be cancelled. Still, all 3 have come positively reviewed by most RS posters whom have used them, and booking a tour which includes the Forum and Palatine is highly recommended as they are more complex sites to understand.