HAAst depends on Asterisk to report the number of calls in progress.  From the Asterisk CLI issue the command “core show calls” and you will see how call are calculated/reported.
If you are running a configuration generator (eg: PIAF, FreePBX) then you will discover that the configuration generator triggers automatic internal (local) calls every X seconds to handle time variable updates and other checks.  (Not a good design as they are loading up your PBX with background “calls”).  There have been an number of discussions about this topic (on various websites) and causing a lot of frustration with users who are seeing their PBX being loaded up with CPU/disk activity to do little more than update a variable.  Since some product vendors only have PHP development experience they must solve every problem using PHP scripts, call files, etc.  Other vendors are quite sophisticated (eg: code in C++/C) and their products (eg: XCally) use minimal system resources.
If you are running only Asterisk then check for channels not being released.  (There are some documented bugs in Asterisk – depending on the version you are running).