iphone EU to iphone UK

i have a query my dad & I are both on Vodaphone. a few months ago I did get an email regarding a text, that should've alerted me into how dad had his phone set up. I had a word and he now has my correct phone number. I just wondered why would a text message go to email?

I can send him a text and he gets them. when he replies/new text it says error in ..states my email'. He has turned iMessage off (he doesn't use Wifi anyway but as Germany is more Mobile data it may try)

any ideas?

thank you so much

dads iphone 5

me iphone 5S