This is the original specification fro the 48A bulb, thanks to Don's Bulbs, now defunct.

The two lamps on the top are original replacement 48A lamps. Note how the metal goes all the way to the left and the glass is longer than the replacement below. All three will work. I do not recommend the 48C lamp that is commonly referenced as a replacement. It uses half the current and is half as bright. Look at the picture of the lit clock at the bottom and you will see the light provided by the 48A lamps is a warm glow.

I am no longer able to source for the 48A lamps.  Many lamp stores incorrectly cross reference the 48A lamp with the 48C lamp.  The 48C are half as bright and do not provide sufficient light to see the dials.

Beware of off-brand replacement 48A lamps as they may be very poor reproductions and will not work correctly.  Be careful and specify the correct lamp and that you want a C5 filament.  Some of the cross referenced bulbs do not provide the correct configuration.  (See my story about why you should avoid Atlanta Bulbs.  They cannot supply the correct bulb and are dishonest.)

The lamps are wired in series. When one lamp burns out, all the lamps stop working. You can test the lamps to see which one is bad and only replace the burned out lamp.

There are some LED replacements for telephone lamps.  LEDs only provide light directly from the LED chip.  Take care that the light provided radiates radially, out from the sides of the glass cylinder.  Additionally the clock will have to be rewired to provide 48v, effectively wired in parallel to each socket.  A transformer will also be required to bring the voltage down from line voltage to 48 volts.

Ben Langlotz also wrote an article about replacing the original lamps with LED strip lights.