No voice during the call between to internal extension

After,that first extension called to other, no voice was hearing. By network analizer it’s seem, that packets of RTP are directed from IP address of extensions to IP address of Asterisk’s server. After restart this issue was resolved, but now I can’t know ,what kind of problem it was. Mabe somebody is able to explaine ?
Thanks to all.

What kind of devices were you trying to connect? What protocol? You need to provide more information.