It could be a friend, could be a telemarker, could be scam caller from overseas.
I myseld don't answer unknown callers with no caller ID or "Private callers". If it is important you will get a voice mail. If no voice mail then it is probably not important.
Unfortunely, there is nothing Fido can do.
Opps. My appologies as I read it incorrectly in my last response.
I have encountered a similar issue last time. After troubleshooting it was caused by a faulty bluetooth headset trying to dial a number when it was in use (one of the push buttons was faulty). Your case might be different.
I would check and see if there are any "rouge" apps and uninstall them from your phone. Sometimes the rouge app will disguise as games or utility app and try to "highjack" your phone. Take a peek in the permission section on those apps and see if anything out of the ordinary is enabled (e.g., a flash light app only needs access to your light control, and does not need access to your contacts, files, and telephone functions).
Sometimes a virus scanner can help locate these rouge apps.
Good luck and keep us updated.