Like you do it any other campaign of your online marketing venture, you need to monitor the performance of calls-to-action and optimize it. You need to evaluate and figure out where to improve and how. To do so you have so many metrics by your side that can help you scrutinize the performance of your CTAs.
Let me explain you about a few primaries and most essential of them.
1) Click-Through Rate
2) Clicks to Submissions Rate
3) Views-to-Submission Rate
Set target for the performance of your different calls-to-action and work in the direction to achieve those targets. For instance to improve the click-through rate of your CTAs you can make changes to your email template and write more compelling CTAs for different offers. You can tweak the language and design of your CTAs for paid ads and so on. It is important to keep an eye on your performance to work immediately where ever you are lacking or if something is missing.