Hello, I went through your query, If your problems with your fingers occur only during specific tasks such as writing, one possibility is what is termed focal task-specific tremor or dystonia (dystonia is a specific type of muscle contraction). The task that could trigger the tremor could include writing, holding a phone in a specific way, holding a cup or other object, basically any specific sustained motion with the limb in a specific position. Performing the same task in the same position will repeatedly trigger the tremor/dystonia after a certain amount of time, but performing the same task in a slightly different position would not. This in your case would be called writing tremor, and your condition would be called primary writing tremor.
Some regard these task specific movements tremors or dystonias depending on the exact nature of the movement. The cause of these types of tremors or dystonias is not known, but they are usually isolated and benign, meaning that progression to a more serious neurologic problem is unlikely.
If your symptoms are severe enough to be bothersome or interfere with your occupation, evaluation by a neurologist, please consult him.