Since the CCIE Voice Written exam assumes familiarity with a variety of unified communications solutions (for example, CUCM, CUCME, unified messaging, CUPS, and UCCX), you would certainly benefit by first successfully completing both the CCNA Voice and CCNP Voice tracks. Then, you could fill in the gaps (for example, UCCX and UC security) through self-study.
Perhaps you have the luxury of working with some of these technologies in your current employment. If so, you might check into the feasibility of constructing a voice lab topology, which can help you prepare for not only the CCIE Voice Written exam but ultimately the CCIE Voice Lab exam.
If you don’t have the good fortune of working with these technologies, you might consider renting rack time from one of the many CCIE training companies. You might consider attending a CCIE Voice Written boot camp class. Or, you might invest in a series of Cisco Press books (as detailed in the next section).
Finally, consider attending a CCIE Voice Written book camp. While I have learned much through self-study, if your budget allows, the benefit I’ve seen from attending a boot camp is that it seems to compress time. Here’s what I mean: when preparing for my first attempt at the CCIE Voice Lab, I attended a week-long boot camp. Upon leaving the boot camp, I remarked that I learned more in that one week than I would have in a month and a half of independent study. So, while I felt that I could have reached that level on my own, it would have taken much longer.