When bowhunting elk by yourself, it helps to NOT call at all. I know it isnt as fun and exciting but ambushing them is many times more productive when by yourself. If you are determind to call then the next best thing to an ambush is a decoy (as was already mentioned.)
Some other tactics that have worked for myself and others is use low light and get agressive. If you can hear your bull bugling and you are sure he has at least a couple of cows you can run right at them and as soon as you see elk, stop and bugle. They are used to running bulls making a ton of noise and if the light is right you can see their light bodies before they can see you. The bull will typically feel you have invaded his personal space and come to you but you had better be close (well under 100 yards) or else he is more likely to gather his cows and move them away from you.
Happy hunting!