If you are using a GSM phone, a 300 mA supply is hopelessly small. It will not work.
When GSM phones are transmitting, which they have to do briefly to register on the network, they take up to 1.9A, for 1/8th of the time, 216 times a second. So if your power supply isn't up to supplying 1.9A, the GSM phone won't work at all.
The average current is about 240mA when transmiting
The pulse is 579us long at 1.9A. If you need a capacitor to drop less than 1/2 volt, it needs to be at least 2200 uF, and more if you are running from the mains and there will be gaps of 1/2 cycle of the mains.
GSM phone supplies are hard work to get right, because of the pulsed nature of the load. That is why ordinary mobile phones won't work from their chargers alone. They need a battery fitted to work, while, for example, laptops don't.