Hi @bertie64,
Welcome back to our Community Forums! Thank you for your post and for your query about your upcoming Home Phone Switchover!
The adaptor you receive should fit into that landline that you currently with no issues. However, if you do experience any problems at all, you can reach back out to us so we can look into this issue further for you and arrange a technician appointment if needed. You can read more about this on our Switchover FAQ page for more details.
Let us know how you get on.
Thank you.