How can I stop crackling noise on Astra landline phone

Unfortunately it sounds like you probably broke a connection in the handset, or maybe the circuit board. Very difficult to find and fix.

If you are into it, take the case apart and look carefully under high magnification at everything inside for visible signs of damage. Take the battery out first.

If you are even more into it, get a can of freeze spray and a heat gun and shoot the freeze spray at various parts of the circuits inside and see if you can get it to make the noise. Also heat it (carefully!) with the heat gun (or hair dryer) and listen for the noise. Using this technique you might be able to find the area that has been damaged. Then look very closely and see if you can find it.

Thing is, once you have found it, you will have to do some micro surgery to fix it. What, exactly, that is, depends on the nature of the damage. You might have to glue the board back together, and resolder joints that were broken.