Sorry, I'm still not sure what you're looking for. You explain you're a little deaf and you can't hear well on your Bluetooth headset so you have to stop the car. Then you say you want to try voice dialing. This confuses me because I'm thinking... What has voice dialing have to do with hearing? It also makes me think... What would voice dialing solve if you still can't hear without stopping the car?
I know it must seem weird to you because you think you're explaining yourself, but you're not. Its just more confusion. If it were me and I was looking for an app to voice dial, I would say
"I'm looking for an app that would allow me to say "call Jane" and the phone would dial Jane's phone number". Just from that one sentence, you would at least know what I'm looking for and what the app has to do.
There are quite a few apps on
Playstore for voice dialing (if that's
all you're looking for). Beyond that, I don't know if they'll work on your phone, I don't know what you've tried, what failures you've seen, and/or why they don't work.
Reading the manual on your phone, it doesn't appear to be very capable in doing much as a smart phone. The price itself leads me to believe it's been built as a limited device and is probably why Medion says it doesn't have the facility to do what you want it to do. In addition, text-speech only means the device can create a synthesized voice to read what's on the screen and voice it to you so you can hear it. It doesn't work the other way around where it can be used by you talking to the phone and making it do things.