The launch is relatively low-key, with the phone being sold online in Europe via a series of flash sales, the first of which will take place next Monday.
Canonical, the company behind Ubuntu, said the limited launch is aimed at "avoiding the pain faced by other platforms" that tried to push their handsets to the mass-market too early.
Aquaris E4.5 Ubuntu Edition: The specs
Processor: Mediatek Quad Core ARM Cortex-A7 processor running at up to 1.3 GHz
Memory: 1GB RAM
Display: 4.5-inch, IPS-LCD screen with 960x540 resolution
Storage: 8GB Flash and MicroSD slot also available
Cameras: Five megapixel front and eight megapixel rear with autofocus and dual flash. Rear camera includes BSI sensors and a Largan lens, 1080P video.
SIM: Unlocked dual SIM to allow owners to use it with their current network.
Colour: Available in black
"We're launching in a very different way and I think that's critical. We can't replicate the mistakes made by other platforms in recent years that have gone for volume on day one by sticking phones on retail and not really giving any thought to what it takes to launch in a mature and cluttered mobile market," said Christian Parrino, Canonical's VP of mobile.
Instead he said the Canonical and its partner bq, a Spanish electronics retailer, would focus on generating buzz for Ubuntu handsets by initially selling a "limited" number to early adopters in Europe online. It will also be giving handsets to a select group of "enthusiastic Ubuntu and bq users" in London today.
Parrino acknowledged that Canonical was entering a mobile market dominated by iOS and Android handsets and would have to work hard to get the Ubuntu Phone noticed, saying it is why it is focusing on selling to a smaller group of early adopters.
"I don't think that somebody whose mental model is 100 percent dedicated to iOS or Android and has no interest in change will go for this type of device at this time."
Roberta Cozza, research director with analyst house Gartner, said the phone would likely struggle to win over users who could buy a similar-priced Android handset.
"I think it is hard to go head-to-head with Android and all that it offers," she said, adding the quality of low-end Android phones is improving.
"An Android device at a comparable or lower price offers a more attractive ecosystem to users in terms of apps and attached services."
She pointed to Mozilla's experience trying to break into mobile markets across the world with handsets based on its mobile Firefox OS, saying it was an example of how difficult it can be to tempt people away from incumbents.
"Even with Firefox OS we haven't seen big numbers at all, yet they were pushed by a major operator and even endorsed by some international brands."
The flash sales will take place over the coming weeks, with the first happening on Monday via bq.com. The date, time and web address for the first sale announced through @Ubuntu and @bqreaders on Twitter, as well as Ubuntu G+ and Ubuntu. Phones will be available across Europe.
Operators such as 3 Sweden, amena.com in Spain, giffgaff in the UK and Portugal Telecom are offering SIM bundles at purchase for buyers in those regions.
What's the phone's USP?
In terms of features Parrino believes the phone's selling point will be Scopes. Scopes are full screen menus that show content stored both on the phone and on services you use. For example, a music Scope would show music on your phone, on the SoundCloud music sharing or the GrooveShark music streaming services and on YouTube, with each section able to be expanded into an app to play music or explore more tracks.
Each Scope can be set as the phone's default screen, with a view to letting users customise the handset around the services and content they access the most.
"On Ubuntu content and services are delivered directly to the screen without hiding them behind apps and the app icon grid," said Parrino.
Apps will also be available on the phone, with Canonical promising official Facebook, Twitter, Cut the Rope, eBay, Amazon, Timeout and Yelp releases, as well as third party Dropbox and Spotify offerings. The only big name missing, according to Parrino is WhatsApp, a gap they hope will be met by the Telegram app.
"We get to come to market with over 1,000 Scopes and apps, some of the leading brands around the world from day one," he said.
Canonical is also hoping to attract developers from more popular mobile platforms with Scopes, which Parrino claims "can be developed at a fraction of the cost of building and maintaining a traditional app".
"Developing Scopes is easier than developing apps since you can essentially use any kind of web API to create an experience on Ubuntu with the Scopes toolkit.
"In a world where a new OS is only as valuable to app developers as the number of users it has, it's a very important factor."
Unlike the Ubuntu Edge, which was sold as a desktop/phone hybrid, the Aquaris E4.5 is more typical smartphone hardware but Parrino said the idea of having a phone power a PC remained "very much part of our future vision" and that Canonical would demo related technology at the upcoming Mobile World Congress conference.
The Aquaris E4.5 will run Ubuntu Touch and Ubuntu 15.04. Canonical continues to work on implementing the same operating system across phones and PCs, said Parrino, adding there should be "major improvements" on realising this longstanding pledge in the next couple of releases of Ubuntu.
"There will come a time when the desktop application and the phone application are the same set of code," he said.
He played down the prospects for Ubuntu for Android, describing it as "a cool conceptual product that doesn't have commercial backing right now from the industry".