I use Turkcell, without problems, for both my mobile and my dongle. There never seems to be a problem with weak signal and lots of people use Turkcell so if you top up with their super tariff - 50 tl - you can call other Turkcell users free. This lasts for one month but you don't lose any credit you have left you are just charged normal call costs until you have used up your credit.
Then you top up again when necessary.