In your binding configuration, there are four timeout values 1) openTimeout as the name implies is the amount of time you're willing to wait when you open the connection to your WCF service. 2)closeTimeout is the amount of time when you close the connection 3) sendTimeout, which says how long the client will wait for a response from your WCF service 4) receiveTimeout is a bit like a mirror for the sendTimeout.
To fix your issue, you need to tweak the most important 'sendTimeout' as:
<bindings>
<basichttpbinding>
<binding name="IncreasedTimeout">
sendTimeout="00:25:00">
</binding>
</basichttpbinding>
</bindings>