Hi there Hudlerock,
Based on what's happening, it sounds like you have call forwarding enabled on your iPhone. This information from Set up call forwarding and call waiting on iPhone shows how to change this carrier setting:
1. Go to Settings
> Phone.
2. Tap any of the following:
* Call Forwarding:
appears in the status bar when call forwarding is on. You must be in range of the cellular network when you set iPhone to forward calls, or calls won’t be forwarded.
On models with Dual SIM, choose a line.
We'd also recommend ensuring that you're not using the same Apple ID. If you are, let's go to Settings > Phone > Calls on Other Devices, and check to see if that is enabled. If you use Wi-Fi calling, check Settings > Phone > Wi-Fi Calling > Add Wi-Fi Calling For Other Devices. These features can result in similar behavior to what you're experiencing.
If this continues after looking over those settings, then we would recommend reaching out to your wireless carrier next. Since this is a feature they support, they would be the best equipped to look into this further with you.
Cheers!