Yes yopu do have ground to stand on - YOUR OWN 2 FEET!
Default 1 - You have told us that YOU submitted the incorrect date of birth so that you could gain this phone. You commited fraud. How the hell can this ever be in any way, shape or form the providers fault when you deliberately and intentionally set out to mislead and illegally obtain goods and credit from them? My very best advice about this one is that you keep shtumm and let it move into history because to awaken interest in this could cost you far more than the value of the debt and default
Default 2 - The debt still will stand and will remain on your credit file till the 6 year anniversary.
You cannot remove a default, even once paid it will still remain on your file unless you can prove to the company that it was placed there incorrectly and as you have already said that it was a phone in your name that you got for a friend who never paid then the debt and default will remain as it is genuine. The provider has absolutely no proof that it was your friend making the phone calls and will assume it was you as you signed the contract.
Default 3 - Seems strange that Lowell have added a second default. This might be worth checking as i thought the default could only go from the original date...but then again this could be a new default if they have added on the cancellation fee, but saying this, they should still have sent correspondence to say the default was to be placed and give you a chance to pay it (not that I think you would have done). But if you have changed addresss since the phone was taken then this could be why they did not contact you. You may be able to question this second default but not the orignal.
Lowell portfolio is a debt recovery company that will hound and chase you for the money. The debt may be at £505 now but you can probably expect it to go higher as they will liklely add their own administration costs on for chasing the debt.
Same as the last debt, you took on the risk for your friend and your friend failed you, but it was the risk in your name and not your friends and so it is you they.
You need to buckle up and just pay the debt (possibly ignore default 1 if you can). paying it will not remove the default or any bad credit markers against your name but it will show slightly better on your credit file and it may mean that when it can start to recover it may do so a little more quickly. So far as all the defaults, they will only come off your credit file after 6 years and not before and nothing you can do will change that. A default is a shining beacon to all credit lenders to show that you have previously not paid your deebts as agreed even though you have been reminded, chased and warned that a default would be placed, this in the lenders eyes is deliberate avoidance of the debt and so they reward you with a big black mark against your name that is visible to other providers and lenders and sternly warns them that you are a bad credit risk.....and as a result you will find it hard to gain credit