Pinpoint not receiving SMS

I've got a UK long code and I can send through Pinpoint & SNS from this number, but any SMS's to this number don't appear to be received.

Two way SMS is enabled and "enable self-managed opt-outs" is left unticked so AWS handle the HELP & STOP keywords. But even when sending SMS's with those keywords, there is no response so would suggest the SMS's aren't even being received.

Any suggestions on how to resolve this?

I've got SNS logging enabled I believe, but can't find how to enable Pinpoint logs for incoming SMS's. All the articles I find are for logging of outgoing messages which appear to be operating correctly.