Just bought a Nexus 5, put a new giffgaff sim in and transferred my number. Everything seemed to be going fine although the transfer updated on my account about 10 hours before it updated my phone.
Number is now transferred (from Orange btw) and I can make/receive calls and send sms but cannot receive them. I did receive one which I believe was sent using Hangouts but others from non-smartphone users aren't arriving.
Any help would be appreciated.
It should work but using Hangouts isn't the most intuitive of experiences.
Try sending a text differently. Open your "People" app (that's the app with the blue and white smiling face).
Chose a contact from the list, yourself for example. Now press on the dark grey rectangle on the right next to your number. Hangouts will open and in the box at the bottom where you type it will say in light grey "Send an SMS message". Now type something in the box.
Send yourself the message. You should get it. I know it works because I just did it.