@adrianpatch You can set up a telegram account using your pc/laptop first. Signing in there may help the phone install. This issue is not specific to 15.04 or 16.04 just a general connection issue. It took me a few attempts to get the number accepted when setting up my OPO. I guess you are but you are using the international prefix for number. So it will just go ahead without reason sometimes.
As for moving to 16.04 yes you can.My bq5 hd us on 16.04. Just put your phone into developer mode in system setting download the ubports installer and follow the on screen instruction. Oh and install ADB tools on your system as well

@themoose007 You can try disconnecting the device, restarting the installer waiting till asked then reconnecting the device. That sometimes works. Can you let us know what device and what installer version.
Edit Also try different usb cables, it does make a difference.

@adrianpatch Yes the Telegram login issue. If you use the search feature on this forum and put in telegram login, you will see this issue is an old not so favorite! For some it suddenly works after days of trying other straight away. Not that that helps you much. Have you tried reinstalling the unofficial telegram app(not the web app) from the openstore. Rebooting the device has helped some as has factory resetting. I never had a problem once I had the code only getting the code through to the device. I have even had to send the code to another device first.
You didn't say which version of ubports your using 15 or 16.04. Installing 16.04 without the wipe option can cause issues. As I said read through the other posts and see what works but basically keep trying and you can always come back here for more info.

@bq10_user Yes it should still work have a good read through. Probably twice to be sure.You will just need to change device name to cooler or frieza and
--channel=ubports-touch/16.04rc --revision=version_number
The version number for you is 9 ,I think to be safe. Any problems please just pop back.

@bq10_user https://forums.ubports.com/topic/1574/august-15-devel-channel-update-renders-your-device-unable
Please read the above thread and follow the instructions. Your device name is frieza for fhd m10 and cooler for hd version.

Nexus5 also ok on 532

@advocatux Welcome back 431 works on my OPO!

@truscellino have just put my OPO back to 428 on the dev channel following the instructions given above by @advocatux using ubuntu-device-flash. You need adb on your pc/laptop and can install ubuntu-device -flash with sudo apt install ubuntu-device-flash. Make sure you have the correct device code name (bacon in my case) and update version that was ok for your device (428 for OPO). So connect your device to pc/laptop start adb then for me it was
ubuntu-device-flash --server=https://system-image.ubports.com touch --device=bacon --channel=ubports-touch/16.04/devel --revision=428
hit enter and let it run. Good luck. Only up now as daft enough to try an update this late at night. Let us know how you get on back tomorrow.

@advocatux white screen of blue circle doom after 5 min. Looks like OPO stuffed up

Just had new update 430 land for OPO