Sonic the Hedgehog Mobile
From Sonic Retro
Sonic the Hedgehog Mobile is a port of Sonic the Hedgehog to the J2ME mobile environment for use on mobile devices, available under the Sega Mobile banner. Depending on the service provider, the game is either provided as one full game or in two parts as Sonic the Hedgehog Part 1 and Sonic the Hedgehog Part 2. Part 1 only provides the first three levels of the game and Part 2 contains the final levels. The game is a port of the 2005 Sonic Cafe game that was simply named Sonic the Hedgehog. 
There has also been a second, not as widely known version the game for Verizon phones, that has everything the original version did, although strangely removing all sound effects from the game. The mobile phone version also came bundled with some Panasonic cell phones in Europe and Japan.
Differences between the Mega Drive and Mobile versions
Sonic the Hedgehog Mobile closely mimics the Mega Drive version, but some features have been removed, most notably the Special Stage. Enemies no longer release small animals when defeated. Graphics have also been resized and the palette of flowers in Green Hill Zone is different. Some of the levels have minor tweaks, such as the removal of a see-saw from the Star Light Zone boss encounter, and the removal of a crusher from the Final Zone boss room.
Some things were added to the game to make it easier, such as unlimited continues and a save feature. A scoring system was also added and the top 5 scores will be saved, alternately they can be uploaded to an online high score ranking. Three levels of difficulty were also added: very easy, easy, and normal. Very easy mode is completely lacking of enemies and only the boss of the Zones are present.
Games secrets can be found at this link.
Part 1 (176x208 version)
Part 2 (176x208 version)