My telephone disconects on the first ring.
Sometimes it's the second or third ring, but it's shorting out on the ring.
What usualy causes this? so that I can fix i

Wayne-

The ringing voltage from the phone company is about 90 volts peak-to-peak,
probably a square wave. There is a DC voltage on the line from batteries at
the phone company, often 48 volts DC positive ground.

From your brief description, it sounds like something in the phone can't
withstand the peak voltage and breaks down. When that happens, the phone
company's equipment senses the DC current and reacts as if the phone had been
answered. However, when the ringing stops, the weak part stops conducting as
if the phone had been hung-up.

It is possible that the phone was damaged by lightning striking a telephone
pole. Look for burnt spots on the phone's circuit board. There may be a diode
bridge used to ensure correct voltage polarity, that has a bad diode. There
may be a capacitor across the line that has too low a breakdown voltage.
Unless you can isolate the weak part(s), a new phone is in order.

Fred