My phone isn’t able to receive incoming calls from my girlfriend’s phone but can make outgoing calls to it
Here is a list of things you could try:


Check your settings Phone > Silence Unknown Caller - disable this if enabled.
Type her number out on the dial pad to see if her number is saved as another contact on your phone that was previously blocked.
Try resetting call blocking software like TrueCaller
Try saving her number under a new contact after removing everything that showed up on Step 2 and adding the new number to favourites.