Google Voice is a 3rd party application, and we have very limited information on it. But, I will do my best to help you out! Hopefully another user here chimes in with more information.
Since you have set the app to ask you how you want to make your calls, that part should be taken care of, as far as your text messages, you will need to send those from the Google Voice app in order for that number to show. You can also take a look at the Google Voice Help Center site for more info!
What plan are you on, a post paid or a pre paid?
Pre paid doesn't allow you to fully use the google voice app with your own real number. Why you ask? Its because they want you to pay for a higher priced plan, and conditional call forwarding is not allowed, no technical reason, financial.
If its post paid (which is really still pre paid for the most part), you can port your number over to them, this will allow you to use your phone number for everything, voice/mms/sms/voicemail.
It won't work as good as the Sprint setup, but you can still choose to use google voice for international calls, it just dials an access number automatically first.
Or don't port it, and use the google voice application for SMS.
This is T-Mobile question not a google voice question.
Sprint allowed me to switch over so that all phone calls and text messages would show Google voice. Is there no way to do this through T-Mobile?
Is there a work around for this?
1. Use the Google Voice app for all text messages
2. Switch my T-Mobile number over to my Google voice number. I loose my old number but everybody calls me on google voice? Don't know if I can use my google voice number as my T-Mobile number on the $30 4g plan?
Sprint is unique in how they fully integrate Google voice into your account.
Since you have the prepaid account, there is no way to use google voice in that same way, as tmobile does not allow conditional call forwarding on prepaid accounts.
What people do in your case is,
1) port your cell number that you want people to use to call/txt you to google voice ($20 fee from google).
2) contact tmobile and have them disable voicemail on your prepaid account.
3) only give out your google voice number to people
4) configure on google.com/voice for that number to ring your tmobile number
5) use google voice for all SMS txt messaging
6) MMS can not use google voice, you have to use the tmobile built in messaging app in your phone, with the tomobile number
7) if you don't use MMS, don't give anyone your tmobile number
8) in the google voice mobile app, configure it so ALL calls go via google voice. So each call you make, it shall call a google access number, almost transparent to you, people will see your google voice number on their caller id.
Helpful enough? If you don't call much, the $30 plan is great, if you have coverage.
I can open the Google Voice Android client, options, more, settings, making calls then choose between Use GV to make all calls, not make any calls, only international or ask every time I make a call. I've had this option since January 2011 on HTC G2 and used this same option on MyTouch4G, MyTouch4Gslide, Sensation & a few others. It has always worked flawlessly. I am on a T-Mobile contract plan.
I can choose either T-Mobile voicemail or Google voice mail and forward calls both ways. GV rings both cell and whatever other number I set & cell forwards to GV on busy or no answer. This way I can take or make a call from my cell or GV phone and a new call to either number will ring the phone I am not on. I can have it ring a VoIP client on my computer also and see the call detail.
PS: When I send a text message the client offers a choice of which number to send from.