After 4 hours of waiting I finally got my AT&T Avail GoPhone functioning perfectly. Here is what it took. After I bought the "Bring Your Own Device" sim from StraightTalk, I called Straight Talk to activate the sim card before inserting it in my new AT&T Avail GoPhone. After the sim was activated the next step was to insert the sim into my phone, turn the phone on, and then edit my phone's APN settings. I first downloaded (via wi-fi on the phone) the free app from the marketplace called "APN Backup and Restore". With this app I made a backup of the phone's APN's and then I deleted them. I then selected the option on my phone to create a new APN. I followed the StraightTalk info that came with the sim card (name, APN, Proxy, etc.). After I entered the APN info I selected the Straight Talk APN i just created, then turned off the phone and then turned it back on again. When the phone started I had the ability to send and receive calls. But, I was not able to send/receive mms multimedia messages or access the internet.
So I called StraightTalk and got some conflicting advice from two agents, neither of which fixed the problem. Then I decided to call again and reached the StraightTalk area that handles the "Bring Your Own Device" program. The StraightTalk agent knew exactly why the internet was not working on my phone. A simple change on his end and the problem was fixed. So I thought the problem had been fixed until I tried to send a mms multimedia message with an attached picture and it just hung there forever trying to send the message. I was getting nowhere so I decided to research the problem and came across another post on this forum. The solution is to modify the APN once more and add "mms" to the very last field of the APN, the APN Type. Be sure the "mms" is lower case and be sure to remove the quotes. It goes like this APN Type mms
I learned a valuable lesson. First, call the direct line to the StraightTalk office that handles the Bring Your Own Device phone and sim cards. Their number is 1-855-222-2355. You can also try calling the standard StraightTalk 800 number and ask to be transferred to the "Bring Your Own Device" office. I could have had my problems resolved within 1 hour if I had spoken to this group in the first place. The agent in that group told me the other StraightTalk personnel should have referred me to them in the first place instead of trying to fix the problem themselves (which didn't work because they didn't know much about the program or sim card.
And thanks to the poster who found the solution to the problem sending/receiving mms picture messages. I hope this all helps others quickly fix their problems.