It says in your website that Ubuntu is a free operating system.
How can I get the source code for "Ubuntu for phones"?
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You can follow the instructions for building Ubuntu Touch from source from this wiki page.
Building from source is quite an involved process, involving getting Android drivers and flashing a mobile device, which is why I'm not going to attempt to summarise it here. Just follow the link, it's a wiki so hopefully it will be kept up-to-date.
If you just want to try developing Ubuntu Touch apps on your desktop, you don't need to build Ubuntu Touch from source, you just need the Ubuntu Touch SDK.