The obvious first choice for using a computer as a telephone would be the market leader, Skype. Unfortunately Skype's technology for call quality doesn't allow sounds from the computer to mix in with the computer's microphone. This means that the voice from an SGD's speaker will not be heard on the other end of a phone conversation.
Skype's solution to this problem is to require using a headset and changing the computer's sound settings to send its voice output to the microphone. A person making or receiving a phone call has to don a headset, plug it in to the SGD, and change the sound settings, then reverse the process at the end of the call. If the phone setup isn't reversed, the SGD's voice won't come out through its speaker, so conversation with people in the room is not possible.
Since I am unable to use my hands, plugging in and unplugging the headset for each call is impossible. Besides, how many calls would a person lose while changing the sound setup to receive them?
A solution I found is Google Voice. Voice allows a person to simply click on the phone number in an email or contact list, or enter a phone number with an on-screen keypad, to place a call. Receiving a call requires getting a Google Voice phone number but is even easier – the SGD sounds a ringing phone and one click connects the call. My voice output is picked up by the SGD's microphone and heard by the other party and that person's voice comes from the external speaker. It is exactly like using a speakerphone.
Google Voice is completely cost-free for calls to/from the U. S. and Canada and pretty cheap for other international calls. It does have some quirks, however. First, you have to have a Google account. If you use Gmail, Google+, or Google Docs, you already have a Google account. If not, getting an account is a simple registration process at google.com.
Also, you need to “tie” the Google Voice phone number to another phone. This could be a mobile or land line, but whenever that phone rings the SGD will ring, too. I tied my home phone land line, which we essentially only use for faxes.
The biggest problem I had was securing a phone number. Google gives a list of what it says are available numbers, but after attempting to select one of these I got a cryptic error message. Several more attempts with different numbers yielded only more error messages, so I took to searching the help forum. One thread indicated that the error message just means that phone number is taken and that it may take many, many tries to find an open number. With that advice I decided to count how many I would try before I had success. That turned out to be 436, and the number I finally got isn't in my normal area code.
Whatever the tedium of getting set up, I now have an SGD that's a fully-functioning phone system, with voicemail, for free. I'll take it.