Any genuine email from Royal Mail which, unusually, wanted you to click on a link, would direct you to royalmail.com, not to unpaid-delivery.com. (That web address was only registered last Wednesday).
Anyway, the vast majority of Royal Mail services (if any) don't require a sender to provide a phone number for the recipient. So, if Royal Mail genuinely had an undelivered package for you, how would they know the number to text? Further, why would they need to know your date of birth? (See the link that they sent you!) It also seems odd that, given they've allegedly got a parcel addressed to you, they need you to tell them your address ;-)
https://personal.help.royalmail.com/app/answers/detail/a_id/6905/~/typical-online-scams-to-look-out-for
https://www.bbc.co.uk/news/business-56496203