If you have trouble receiving the SMS messages sent by email, firstly look for a "call me" option and make sure the web phone or app is open and wait for the automated phone call to arrive whereby you can verify via phone call.
If you cannot use that option, it is possible eBay is having trouble sending SMS verification messages. You can reach out to them directly via this link:
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
Normally they will be able to authenticate you quickly and resolve the issue.
