Unfortunately, MTBF is not a practical or reliable measurement in modern servers. The all concept of MTBF is that if a specific model / configuration is being used by many over long time, we can likely know its reliability.
Today, most of us happily trade potential extra reliability for proved extra performance and power efficiency. For example, would you build your new servers on 18-24 months old hardware just because it proved its reliablility? or just go with the last generation of CPUs with more cores, horsepower and power efficiency?
Also, unlike old-school telephony systems, systems are quite customized, and of course, heavily reliant on software. How reliable is BIOS version x.xx or driver version y.yyy? Is the latest OS/DB/app server patches increases stability or does it have stability regressions? How many servers in the world actually use the same exact mixture of hardware/ stack version as you?
If you need high availability, you will anyway need to add redundancy to your system (dual-everything, clustering, hot spares, DRP, what have you). So, the relative reliability of each hardware component is typically not a significant factor, as you build you infrastructure to survive single components failures. Just live with the uncertainty (reliability is retroactive) and plan accordingly.