Can I install 'whats app' messenger into the Doro 6520?

Fix your phone time and if pereists install a new software to remove the corruption of data.

I believe it indicates a loose memory card, but I didn't see it in the manual.

Free manual from the manufacturer's web site:

https://www.doro.co.uk/doro-6520.html

Start by going to “Contacts” and tap on the contact you want to block. Then, press the menu key at the top left of your Doro 6520. You will see a menu where you will select “Block Number” or “Add to Auto Reject List”. The title may vary depending on the model you own

This site seems to address the problem:

http://deviceguides.vodafone.ie/web/doro-6520-proprietary-os/explore-your-phone/internet/set-up-your-phone-for-internet

Follow the instructions on the screen to set up your phone and prepare it for use.

1. Find "Reset all" Press the Navigation key. Select Settings. Select Security. ...
2. Restore factory default settings. Key in the phone lock code (default is 1234) and press the Navigation key. Press the Navigation key to confirm.

Doro 6520 - Restore factory default settings - Device Guides

https://userguide.vodafone.co.nz/doro/6520...os/basic.../restore-factory-default-settings/.
or go to this link might help.
How to reset clock on 6520 mobile Google Search

Hello, you can try "phone to phone transfer" tool, then you can transfer your photos between two doro phones.
This method is very easy and effecient, so you don't to waste a lot time to do this thing, you can finish this thing in a fast time.

Click the following words to know more details about this way:

Samsung Data Transfer

One Click to Transfer Files from Android to Android

6520 reset network

Unfortunately sir doro 6520 don't have this feature...