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.
SOLVED
If you have a Windows 7 computer, go to /windows/fonts and copy the Calibri font to a USB stick or SDHC card. Insert the card or disk into the Linux computer. Click on the Calibri file - you will be given the option to "install". Click on "install" - "install" will change to "installed". Repeat for all versions of the font. Job done and LibreOffice will recognize the font. Any files with this font will open with the correct font. This works on Ubuntu 13.1 and LibreOffice 4.1.3.2
I would not advice that. In addition to be a license problem, it will impose additional problems. IMHO it is far better to use google's crosextra-carlito and crosextra-caladea fonts and make some substitutions in libreoffice/oo. See wiki.debian.org/SubstitutingCalibriAndCambriaFonts for the complete details.