Here are the facts first:
1) I live in an area with mediocre Verizon cellular coverage; (some calls complete, some fail, the rest break up.)
2) I've used a Verizon Network Extender for 4-5 years with an iPhone 4S and the results have been positive--except for the range, which is poor and restrictive (I live in an apartment and can only make/receive calls in a small zone near the Extender.)
3) I recently upgraded to an iPhone SE, which supports Wi-Fi Calling. I turned off the Network Extender.
4) I activated Wi-Fi Calling, which works--until the phone detects the unreliable cellular signal, switches over, and typically ends the phone call. To try and circumvent this, I put the phone in airplane mode (cellular off) with Wi-Fi turned on. This activates Wi-Fi Calling, which is usable... until it inexplicably cuts out during a conversation.
5) I have detected no problems with my Wi-Fi network. My internet connection is reliable.
Question: Why might this be happening, and what should I do about it?
Any help is appreciated.