When I don't have 3G etc iMessages only go to my iPad, and never come though on my phone

so when I'm out and text ppl, not using iMessage because I don't have 3G etc, I never get a reply from them as they obviously are on wifi or 3G so When they reply it's as an iMessage and it comes through on my iPad at home. So I don't get there reply till I'm home.

dont know how to fix this?? It's only started to happen recently