I had a lot of trouble with my DynaVox Max at first, and it would lock up and I couldn't turn it off. Because the battery charges all the time it's on, I had to wait several days for it to run down so I could turn it off. The sales rep had to come out numerous times. Usually rebooting would fix it, but once it was the setting "phone type" that was impeding it. Now I just leave it on the phone screen, and it allows me to communicate over the phone pretty well. I can also communicate in person with it, but prefer the LightWriter for that, as it's portable.
The big problem for me is that the DynaVox speech is too fast and sometimes overwrites itself. I can adjust the speech speed, of course, but then the canned responses come out at a glacial pace. If I set the speed so that pre-programmed phrases sound normal, it is too fast for conversational speech that I type in. I have to spell a lot of things phonetically, and the voice is fairly limited. With pre-programmed phrases, I can reword them until they are understandable, but in conversation, having to go phonetic really REALLY slows things down.
My best bet in conversation is using two systems ... dynavox for longer thoughts, LightWriter for quick comments to keep the conversation moving. I also have a TTY phone (for the deaf, sending text messages) for emergencies, but haven't had to use it yet.
BUT ... compared to not being able to communicate at all? Being able to answer a phone call or place one? Priceless. And I can convert to eyegaze use when/if needed. Also have it unlocked, so can use as a PC.