This is reviving an old question, but I'll chime in...
Just clarify, with an iPhone, you can only send and receive texts over wifi that are sent via iMessage, that's an important point because if someone with a non-iPhone were to send you a message, you would still need the mobile network to retrieve it. iMessage is a great service because it can use your phone number to identify you on their network, making it very easy for iPhone users to seamlessly use the service...but then again it's that same seamlessness that causes a lot of problems for folks after switching away from the iPhone, so it has its disadvantages as well.
I have the same problem where I work, where I have zero to no cell reception at all. I've just let me friends know that they need either use Hangouts or email me because I'm simply not going to get their SMS texts in a timely fashion during the day. Most of friends that took my advice and started using Hangouts really like it and continue to use it all the time anyway.