Blocked number help

[Hanksride]

There is a longer story to this but to keep it simple I did have an iPhone 4s and I may have blocked a phone number that I now don't want blocked however I have sense upgraded to iphone5s. The number is not listed in my blocked numbers. When the person tries to text me they keep getting an "undelivered" message back. I can send them a message but can't get one back from them. Anything I can check or do??