1. Train your agents well and often.
2. Reward your agents for solving issues quickly, but mainly for good customer reviews - you don't want the speed affecting quality, which will cause a loss of clients.
3. Analyze the calls to see which issues take the longest and then either train your agents to deal with this issues as best possible, or create a smaller unit of agents who only deal with these issues (which will make them more professional = quicker turn-around times).
4. Ask your agents what would help them improve the speed - they know best + answer some calls yourself, to see what takes the longest and why.
5. If your system supports this, include an option for customers to select a 'call back' within x hours for cases which are less urgent (this will reduce the pressure during the peak hours and allow better service).
I'm happy to help you through the process if you like.