I just read TSB XJ307-06 and I'm shocked by this part of the text:

10. Select and run ‘re-configure existing TCM’ application using the ‘Coded Access’ route.
Write down the 10 digit alphanumeric code that the WDS displays. Since this code will change over time, have the VIN ready and immediately contact the Technical HelpLine at 1-888-JAG=DLRS, option 2, then ZF option 2.
Be prepared to write down the code you are given and then enter it on the WDS.

Does this mean, you have to phone up the helpline in the middle of a flash of the TCM ?
( I'm really shocked by this idea.)

That is a bit peculiar. And probably impossible for someone in Germany to call a U.S. 1-888 number. Talk about getting seriously hosed. You can't back out of the flash, and you can't call to get the access code.

Is this coded access because the reader could not provide the number of the TCM?

TSB's are released in different languages and are country specific. The number of the TSB mentioned is a US format one so has a US phone number. The UK one will have a different number etc.

I believe Jaguar often use the coded access so that they can track when a specific update is being done or to make sure it is valid that it is done.

My main concern is not to reach the appropriate number for my country, but the dependency on a phone service to do service on my car.
For DIY's, who are a large part of the jag community, this is not very nice.

Hopefully the next XJ doesn't require a 24/7 online link to permanently validate its licenses...