The arrow indication signifies that the device has an IMS bearer and registration (VoLTE). I'm assuming that you have disabled calls over that bearer, meaning that when you make a call now you see 3G in the notification bar, and when you hang up you see the phone eventually return to 4G and then the arrows re-appear?
The phone is initiating the pdn connection via the IMS APN (see settings-> more networks -> mobile networks -> access point names... you should see one called IMS. I suggest that you don't delete it or modify it.
The advantage of VoLTE is that it's delivered over the 4G frequencies, such as 800Mhz, therefore you get better coverage (indoor too) compared to the 3G frequencies.