That can also happen if the phone switches between network types when calling - e.g. for a long time you couldn't call over LTE, so your phone switched to UMTS (3G) or GSM (2G) network to make and receive phone calls.
So you could have excellent service for data and still lose signal for calls.
That makes sense too. I thought that also at that time that LTE was relatively new, so in theory, the UMTS or GSM would have a stronger signal than LTE, which would have meant my calls would be fine.
So you could have excellent service for data and still lose signal for calls.