1. Open MicroB - the browser your N900 comes with - it's just labeled "Web".
2. Type www.jumblo.com
2a. If you've changed your browser settings to not always accept cookies, you'll have to accept them, most likely. I don't know, I didn't experiment.
3. Use the www.jumblo.com site like you would from a computer.

It shouldn't be harder than that (I don't have an account on jumblo.com, and I didn't actually try to make a call, but the site loaded and seemed fine). The great thing about the N900 is you don't NEED apps for stuff like this. Now, an app might be preferable, but it's not necessary.

If you try it and it doesn't work though, do let us know.