If your solar panel is rated at 300mA then it is when it is pointing directly at the sun at noon. Its current will be much less as the sun moves and as the sun is at an angle during most of the day.
Why do you have a voltage regulator? The charger for your mobile phone might be an entire charging circuit, not just a regulated power supply.
Different battery chemistry requires different charging methods to avoid a fire and to let the battery last a long time. Which type of battery is in your mobile phone? Ni-Cad, Ni-MH or Lithium?
Why do you have a rechargable AA cell?