it’s copied and pasted but it’s the same for wifi:
According to Anandtech, the iPhone has an amplitude of measurement of the signal which goes from – 51 dB (maximum quality signal) to -113 dB, low value which corresponds to the loss of the connection. In other words, if your iPhone receives a signal between these two values, you are able to call. If you are above -51 (therefore, -40, -30 etc.), the connection goes beyond understanding and you are undoubtedly in space, or in a parallel dimension. If you are around -110 dB, you are able to make calls, but the connection is poor.