No service alert

Existence of coverage and the signal strength are indicated by the four vertical bars in the top right hand corner of your screen. Additional information on the type of service, eg 4G, may be displayed where the Wi-Fi symbol is shown here:

I am not aware of an alert through loss of signal such as you describe. Maybe someone else can verify this. I just assume that there is coverage unless the icon on my screen indicates otherwise. I am wondering why you would need an alert. If you lose service during a call, it will be immediately obvious. If service is lost between calls, the icon will indicate this.

Apple doesn’t offer an alert when signal is lost. That’s something you might want to suggest to Apple→Product Feedback - Apple. It sounds like a useful feature.

I understand, thanks for the clarification. I was about to suggest what Mr Finch has said, but he beat me to it!