I got Swirley to work on a 3G AT&T 2.1 Firmware without tampering the iphone IMEI # and i still have visual voicemail.
What you need
1) AT&T RAZR (only cause i know it works b/c thats what I used)
2) Must be registered to ATT.COM, where you can view your account pay bills etc for your wireless account!!
My Steps
1) Take sim out of your iphone and place it into the razr. Make a quick phone call and wait 15 minutes
2) Go to webpage http://mymedianet.wireless.att.com/ put your number in and you will get a text and it will get you into media net.
3) Once in medianet look for the option to Register for an account when you complete registration you will get emails thank you for registering with medianet.
4) Go back to http://mymedianet.wireless.att.com/ and click the picture of the cellphone so it saves settings
5) Log into your att account and click features. Currently you should be checked for iphone text plan but know you have an option for AT&T messaging. I had IPHONE UNLIMITED TEXT 20$ I changed it to Messaging Unlimited 20$ (its in orange AT&T reccomnds) once you clicked it make sure you save settings it will tell you it has been updated immediatley.
6) Called att have them check snooper for MMS and WAP push and also have them check a feature called "premium messaging" its there if they dont see it get another rep.
7) Once they do that get off the phone aznd put your sim finally back into the iphone.
8) open Swirly the onlt settings i used are MMSC is mmsc.cingular.com Proxy 066.209.011.032 Pic MAX Agent Iphone save wait for the green M and then close Swirly
9) call at&t have them look into snooper for mms and wap push but now ask them to send "over the air sim update"
10) once they say it has been sent turn off you phone hang up with att and wait a 1 minute.
11)Open swirly wait for the green m now hold the home key until swirly closes and dont let go of the home key and press the power key too. Once the phone shuts off turn the phone back on
12) open swirly wait for the green m try sending one, mine crashed on the first but worked on the second attempt
Its a 12 step process hope you have success like I did!!!