Cell phones are mostly reliable. Key word being "mostly".
But the reason doctors on call carry pagers is because they are far more reliable.
For some things, the older technology is the best.
And there is also the fact that a cell phone eavesdrops on you and a pager does not.
Pagers use radio signals that are stronger than cell signals so they are useful in an area with less technological investment in towers. Secondly, the reason drug dealers and terrorists like them is that you call back using a land line which is less susceptible to being intercepted.