they will either accept a letter from your uncle stating the item was his personal property to give to you and he will have no finiancial gain from giving you this gift
Doubtful. If the uncle was sending his own phone to himself, he might get away with that but I've been asked in the past to pay tax on my own possessions.
They don't want to release it because the tax on it hasn't been paid. Given that there is no faktura, you can write them a letter stating what the value of the item is and asking them to calculate the tax you owe them based on that amount (hint: don't make the amount too low but obviously don't make it high either).