Why does iphone 6 always send text messages when others are on the phone?

If the phone can't receive the call, directly enter the SMS call, indicating that the phone has set up call forwarding.

If the phone can't receive the call, directly enter the SMS call, indicating that the phone has set up call forwarding;

Call forwarding generally includes power off or out of service area, transfer in case of busy, transfer all, etc.

If call forwarding is set, just cancel the call forwarding function directly. You can operate it through the phone menu Settings and Settings-Phone-Call Forwarding.