HAae you recently changed a number, maybe brought a number to giffgaff?
Can you get the person whose texts you cannot receive to delete you entirely from their contacts and get them to check to see if they have multiple contacts on their phone for you. If they have, get them to delete all of them and them re-create a brand new contact for you, with the correct contact details.
If the issue is only linked to a sender of text using an iPhone, then this may resolve this (especially if you have changed a number previously and that person still has the old number on a contact for you.
Hope this make some kind of sense. I have an iPhone and have had to do similar myself