>> No rocket science needed there.
>> Bridge rectifier and a capacitor saves it for you.
>> If you want rocket science you can add an inverter at this point.
>
>What would I use an inverter for?

Just think about Russell said. A bridge rectifier and capacitor is going to
store the rectified ringing voltage on the capacitor. At 90VAC this is going
to be about 110-120Vdc on the capacitor. Now use a switching regulator
(invertor) to drop this to your 5V. If you can get the current draw at the
input to the switcher down to the 400uA range, then a reasonable size
capacitor will keep your micro running for a surprisingly long time.

However, a better way of doing things may be to forget about using the ring
voltage. You will need to protect against it though. Use a buck-boost
regulator to work on the normal dc voltage on the line. This type of
regulator is more complex than a simple buck type, but it does allow the
input voltage to go below the out voltage, and boosts it to the required
voltage. This would keep your circuit working in the off hook state.

You will need to decide precisely which mode of operation you need. It may
be acceptable for the display to vanish some time after the phone is lifted
off hook, and if so then you probably need to have some form of input
voltage detection so you can deliberately blank the display before the
voltage gets that low that the display just fades out, which may look
strange to the customer/operator. Using the buck/boost regulator may allow
you to keep the display on permanently, which could be a preferable way of
doing it. You need to think about this.

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