Google Voice vs LG Cosmos

We use Google Voice at work for our on-call nurses. We used to use a third-party call center with live people to handle this. We would tell them each day what nurse was on call, and what message to say. They, more often than not, got it wrong, and we continued to pay for it (the bill to the third-party, and the lost revenue from a patient referral). When I learned about Google Voice, I suggested we give it a go and test it. They were so pleased with it that we canned the call center after only a week. I instructed the clerks how to change numbers and specify which numbers to forward to. And I haven't heard anything out of them for about a year or more.

Until today. We recently hired a new nurse and they were trying to enter her phone number into the list of available numbers to forward to. When you add a number to Google Voice, GV will call the phone number entered and ask you to dial on that phone two digits that are displayed on the computer screen, to avoid abuse of their system. I've never had any issue with this, but this new nurse's phone, an LG Cosmos, would not pass verification.

I suspect the problem was that GV could not properly hear the DTMF ("touch tone") tones. I adjusted the DTMF settings on her phone so that the tones would play longer. No go. I tried using her phone in speakerphone mode. I even tried using a website to play back the DTMF tones into the phone's microphone. Still nothing.

I then tried having GV verify my own personal cell phone. Worked the first time. Both the LG and my phone are both on Verizon. What's the deal? I started doing what I always do when I have no more ideas, Googling for answers, and found more similar complaints than actual answers. Then I ran across a post from someone who had an iPhone 4GS that couldn't get GV to verify the phone, but this person also had a fix - turn up the ringer volume. This seemed strange to me - why should the ringer volume affect the loudness of the DTMF tones? I tried turning up the ringer on the LG and it still failed. But now that I was in the land of trying-things-that-didn't-make-sense, I wondered what would change if the phone thought a headset was attached?

The LG has a standard headphone jack, so I just plugged in my earbuds and tried to verify again. Success! I'm not sure if this a failure of the phone, or Google Voice not being sensitive enough. I don't like the fact that there are no other ways to verify a number, and that I had to resort to a weird hack like this. Still, their service is free, and for a small business like ours, it works better than what we *were* using.