Android has two mechanisms that claim memory back from the applications that you run.
One mechanism is called OOM and is an integral part of linux. It activates only when the memory is really needed for something else. A phone call would normally NOT trigger this mechanism. Playing a game would, or copying large amounts of files from internal storage to SDCARD.
The other mechanism is native to android, and closes applications that have not recently been used. The exact defintion of "recently" is a moving target. It changes with every release of android, and also with the specific modifications that the manufacturer has made to it. Only the application that is currently visible is always recent enough to not be killed under this mechanism, on every version of android.
Apparently on your system and phonecall is enough to trigger that.
You can try two things to avoid this to happen. One is to switch back to Chrome while you talk on the phone. Use the multitasking functionality of your phone to do that. Chrome will then be the most recent app and android should respect that even during long calls. Note that your call wont be killed either under the same mechanism, because it handles audible interaction, which also makes it a currently used app even when not visible.
The other thing to try is an app that I made. Its originally designed to solve this very same problem for the Opera Mobile browser (now called Opera Classic). Its flexible enough to let you use it with Chrome. However I dont know if it really helps, your mileage may vary. You can download it for free here: http://forum.xda-developers.com/showthread.php?t=2269810
There is also another aspect to your problem, namely memory management inside Chrome (not android). I vaguely recall that Chrome can evict tabs from memory when you look at other tabs. If that is true, the above tricks might only help you to keep Chrome itself alive but the previously loaded content may still be gone. In this case, only a new improved version of Chrome can solve your problem, or switching to other browsers.
I know that Opera Classic does not evict single tabs from memory, neither does Opera Mini. They either stay 100% in memory, or they are killed 100% at once. If you dont like those two, you may want to test other browsers specifically for this property and then apply one of the mentioned tricks to keep them always in memory.