Dear,
Don't go on price. DKU cables are compatible for Nokia 3125. The phone works as per suit specification. You can not d/l ringtones to 3125. There is no room for it.
But all other features, synchronize, application upload are working nicely. Get latest PC Suit from nokia web site. Remove all previous versions from PC then install the latest.
As soon as you connect the phone with PC (DKU Serial as detected, although it is USB) the small icon sitting near clock pops up and notifies that NNNN Model connected.
You can use all features available as per model number.
TC