Do you have any evidence of that behavior? Earlier in this thread, I asked someone to check the contents of the file That file keeps the history of numbers that were dialed.

I am very interested if the application is really concatenating numbers together, but I don't think that's possible, knowing how the code works.
I think that behavior is coming from Grandcentral.com.

Please remember that the Grandcentral Dailer program actually does very little...it gives you a friendly way to enter a phone number, then submits it to grandcentral.com to place the call. It's up to grandcentral.com and your VOIP provider (such as gizmo) and your VOIP software on the tablet to handle all the other details, such as dialling, volume, etc.

If you are hearing a busy signal, then grandcentral.com has placed a call to a number that is busy.

I am very interested in diagnosing any errors in the number that Grandcentral Dialer is submitting to grandcentral.com, but anything beyond that point is out of my control.
Actually, if you're having a consistent, reproducible problem, having the real phone number will help me with the diagnosis. Feel free to send me the details and a copy of history file (/home/user/.gcdialerhist) via e-mail if you don't want to post them publically. I will honor your privacy.