You might have grabbed MS’s Powerpoint Viewer which includes the fonts (for “free”), although the terms of use include:
You may use the fonts that accompany the PowerPoint Viewer only to display and print content from a device running a Microsoft Windows operating system.
My hunch is that MS won’t know that you’ve installed them under Ubuntu (or is your machine dual-booting?), but…
… If you’re forced to use alternatives, these would be my recommendations:
Calibri alternatives: either Asap or Gandhi Sans would do. The differences from Calibri are more pronounced in the italics.
Cambria alternatives are trickier: but either Droid Serif or PT Serif would get you most of the way there. Droid and Cambria share a designer in Steve Matteson, so that accounts for some “family feel”.
Or just go with new choices altogether!
(PT Serif + PT Sans? Linux Libertine + Biolinum? Gentium Book + Source Sans Pro?) The problem in that case is that you can’t be sure that they’re installed on other people’s systems if you’re sharing files.
No doubt about it: it’s a pain.