Convert a ringing telephone voltage of 90V AC to 3.3V 1000mAh DC current so that it only triggers when the telephone is ringing?
What are you trying to do? You can step down voltages (90 to 3.3), and you can rectify alternating current to direct current using a bridge rectifier. Mentioning a current capacity like "1000 mAh" suggests you are using a battery or trying to replace a battery.

Batteries are typically rated with current capacities to show how much energy they can deliver over time. A 1000 mAh (or 1 amp-hour) battery would in theory be able to deliver 1A for one hour, or 0.5A for two hours, and so on. Typically they are derated as the current being drawn increases. Thus, 1 Ah battery may supply 100 mA for ten hours, but can only sustain 1A for a few minutes. Check the battery datasheet.

If you've simply meant milliamperes (mA) instead of milliamp-hours (mAh), then this means you need to be able to supply 1000 mA at 3.3V DC.

You can use a step-down regulator, also known as a buck converter, to convert 90V to 3.3V. No converter is 100% efficient, so you will need to have some overhead.

A bridge rectifier can convert AC to DC. There is an associated voltage drop, so you will need to account for that.

Assuming that you need 1A at 3.3V, that is 3.3 watts. Therefore you will need a bit more on the input side to account for inefficiencies. Using Ohm's Law, the current on the input side (\$I = \frac{P}{E}\$) will need to be more than 37 mA.