iTunes accounts/AppleIDs are not and never were designed to be shared between individuals. Accept it. If you do it anyway, you will most likely have problems with your devices. I agree that “calls on other devices” is likely the culprit here. Separating AppleIDs will resolve the problems with calls, texts, and other odd behaviors.
As for the “iCloud full” messages, that may be due to iCloud backups being enabled, with a lot of content on the phone (usually photos/videos). These will typically be backed up along with the phone, unless you specifically disable photo backups in the iCloud Backup settings on the phone. Apple only provides 5GB of free iCloud space, and that is easy to fill with not many photos and videos. Alternatively, you can periodically backup the phone to iTunes on your PC.
JKG