Unable to receive phone calls or texts

I ported my number 3 months ago to GiffGaff and ever since then when I put my sim into any iphone I can't recieve any phone calls. When I put the same sim into an Samsung Galaxy S3 it works perfectly fine. I have gone back to GiffGaff and they have done everything they can, I don't think there is a problem with them, I think the problem lies with apple. When I go into Messages tab on my iPhone 5 it says "waiting for activation...."

Can someone please help me resolve the problem