Hi there @Fellsider, welcome to our forum and thanks for your post.
Sorry to see you are having the issue with the particular mobile calling the landline. It certainly is a bit of an odd one.
Are you able to try the sim card of the mobile you are ringing in another phone to see if the issue is the same? It may also be good to check to see if there is any further advice here. If you still see this issue after checking these few things please let us know.
Regards
Nathan