Which phases of SDLC does the testers begin to write test cases?

Testing is one of the most important components of the software development lifecycle, but when to begin testing depends on a number of factors.

For the betterment, reliability and performance of an Information System, it is always better to involve the Testing team right from the beginning of the Requirement Analysis phase. The active involvement of the testing team will give the testers a clear vision of the functionality of the system by which we can expect a better quality and error-free product.

Once the Development Team-lead analyzes the requirements, he will prepare the System Requirement Specification, Requirement Traceability Matrix. After that he will schedule a meeting with the Testing Team (Test Lead and Tester chosen for that project). The Development Team-lead will explain regarding the Project, the total schedule of modules, Deliverable and Versions.
The involvement of Testing team will start from here. Test Lead will prepare the Test Strategy and Test Plan, which is the scheduler for entire testing process. Generally Organizations follow the V – Model for their development and testing.

So, In SDLC after completion of FRS (functional requirement specification) document, the test lead prepare the use case document and test plan document, then the tester role is start.