One thing would be to check your apps to see which ones have the permission to make calls. There are apps that can scan permissions and produce lists of apps with each (I have one called Addons Detector, which I installed at the point when a particular type of intrusive advertising, since banned, was becoming popular with apps, which can do this). You may even be able to find this from the system menus, depending on what phone and Android version you have: I can find it from the advanced section of my phone's Apps settings.
Of course there may be a lot of them, but if you can narrow the list down this way, and perhaps identify some that have no conceivable legit reason to have it, it may help pin down the culprit.
If there is a pattern to where, when or what sort of number that might help too. If they happen at certain times and you can afford to put the phone in airplane mode or turn it off at those times that would confirm that they are really coming from your phone, rather than an error in the carrier's system or someone cloning something (just trying to consider all possibilities here).
If you can live with only pre-installed apps and run in safe mode for a few days (depending on how frequent this is) that would help check whether it's an app you installed: they can't run in safe mode, so if that stops it that would indicate that it's an app you installed.