Report it to your service provider immediately. They should have a facility to register calls you identify as nuisance calls with a view to taking that evidence to the police, and they can also identify the caller following a lawful request for information by law enforcement officers. There will be a threshold to pass to define a nuisance; several in one evening should qualify.
The calls you describe are coming from a fixed phone, which in many territories by default has no caller ID, or a mobile phone with call ID turned off.
Also, keep a record of the calls both on paper and on your iPhone.