A few weeks ago I started using 02081802727 which is the Voipbuster access number from the UK if you wish to use your mobile to get free calls using your voipbuster account.

Since a few days now, calling it with my O2 number automatically hangs up. Is this O2 blocking my calls to this?

Yes, I'm sure I read about O2 planning to block voip a few weeks back.

That is ridiculous if it's true. Are you sure they weren't going to block ougoing VOIP calls from O2's 3G network i.e making voip calls using O2's data connection.
A while ago calls from Orange were blocked to one of my VOIP numbers and it turned out to be an Orange routing problem. If O2 are blocking calls to VOIP numbers I would drop Ofcom a line, it sounds like it might be breaking some rule or other.

I'm not sure I tried to find the thread, but I can't find it. I know it was something to do with O2 blocking voip.

Sorry totaly mis-read your post didn't notice you said an access number for getting cheaper calls from a mobile. It's standard practice for mobile networks to block these numbers.

Just called that number from an o2 PAYG phone.

I got the finarea-SA voice saying I do not have enough credit / i am not registered so I guess I can call it fine.

Mobile operators don't want people using VOIP over cheaper data links.

T-Mobile offer a web'n'walk package with a high limit but have apparently blokced VOIP and IM (although given their Sidekick II comes with AIM built in that seems slightly contradictory )

If you're serious about VOIP over a wireless network you could always try Linksys's new WRT54G3G router with a Vodafone tariff

So I guess it is finarea blocking my number.

I was able to use it once, where I made a 30 minute phone call. Since then, haven't been able to use it.

I wonder why?