Mobile Number Not Associated With My Phone

I received the message below in iMessage. 
 

I transferred my old number when I took out an ID Mobile 30 day rolling contract (i renews in 4 days time. Despite the message saying the number is not associated with my phone, it is listed in My Device Settings. Is the message displaying because my current 30 day rolling contract is coming to an end before it renews?  When I first received my ID Mobile sim, it had a different mobile number until I entered the PAC number from my previous provider and this number transferred across.