This is an ongoing problem I have had for the last year. There have been several updates to the IOS GoToMeeting app over this period and I have owned three different models of iPhones. Yet the problem continues.
If you select the "Dial In" option from the app it successfully callls into the meeting but the iPhone microphone does not transmit sound to the meeting.
The workaround is to quit the app then go to recent calls and dial back in to the meeting. After you do this the microphone works.
At times I found that if I did not quit the app the microphone continued to not work for any cellular phone call.