This can happen if your PBP Memory Limit variable is set too low. If this occurs, the PHP thread is killed before it has a chance to display anything on the screen, so it is not even possible to display an error message.
In the Apache error logfile /var/log/apache2/error_log you will see entries similar to:
Allowed memory size of 8388608 bytes exhausted (tried to allocate 64 bytes)
For those of you that never read manuals, here is an excerpt of the Phone Book Installation Manual:
Be sure to set the memory_limit from the default of 8 MB to at least 20 iMB n /etc/php.ini to avoid running into PHP memory problems as the CDR file size grows. The currently needed PHP memory can be viewed in the Tools menu.
You can viiew how much PHP memory is actually needed in the Tools Menu of GEOTEK Phone Book. The amount of memory is mostly dependent on the cdr_maxsize setting in config.php, which is set to 1200 by default. As long as you do not disable the auto-archiving function of Phone Book, the php memory consumption will never exceed a specific value, no matter how many calls you make.