Cant receive text messages from a certain number

Hi i have the galaxy s5 my friend has a payg vodafone smart ultra 6. Up until a couple of days ago we could yext each other, now i cant receive his texts. I can receive texts from everyone else exept him! I can send him texts and call him he can also call me just cant send texts to me. Help please. Ive been on loads of forums and live chats to try and work it out but not resolved as yet and cant work out if problem is my end or his end

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