Call Back option is having issues, as you no doubt have found out.

Some have had success when using the link below and using a phone.

Others have found if a bot is answering,  to keep entering  '  I wish to speak to an agent'  you may need to do this several times before being passed over to a ' live agent'

Here's the link:

https://www.ebay.co.uk/help/eua?id=5190

@suncity1967