Hi Max, the Nokia fuse is 2A and not 20A. Anyway if you think this fuse can also work on Iphone this is a good starting point. I suppose that 3G and 3GS specs are the same, except the physical size.
Based on these specs the appropriate part number (Littelfuse Inc) seems to be
0435002 for 3G and
0467002 for 3GS.
A small note about 3GS: the above fuse size is 0603, a little smaller of the original. The correct size is 0805 (2012 Metric), but unfortunately Littelfuse doesn't have this size available. An alternative could be
MFU08052 from Vishay/Beyschlag Inc.
The fuses mentioned here are standard ones while the original ones are resettable.
I tried to find resettable ones, but it's very hard because the specs are totally different.
Anyone experienced can confirm that the above parts are good for replacement?
I want to avoid leaving readers with the doubt if they are good or wrong.
Many thanks