There are several open source authentication servers out there. E.g. I am working on privacyidea (http://www.privacyidea.org).
There is one open hardware project I know of. It is the nitrokey (http://nitrokey.com), which is also supported by privacyIDEA starting in the next version of privacyidea.
But this is also a USB device similar to yubikey.
There is not open OTP display device I know of.
Also, Most of these devices come preseeded, which means the vendor generated the secret key.
But if the only concern is your phone - you can use the yubikey with an USB OTG. Or there is the yubikey neo, which works via NFC.
There is one open hardware project I know of. It is the nitrokey (http://nitrokey.com), which is also supported by privacyIDEA starting in the next version of privacyidea. But this is also a USB device similar to yubikey.
There is not open OTP display device I know of. Also, Most of these devices come preseeded, which means the vendor generated the secret key. But if the only concern is your phone - you can use the yubikey with an USB OTG. Or there is the yubikey neo, which works via NFC.