Phone wont stop the activation screen

Request unlock from att on the phone or online.
https://www.att.com/deviceunlock/client/en_US/termsAndCondition?

Let me start by saying I'm not an expert, but after going through a lot of gyrations I came up with a solution that worked for me. I'm sharing it here because I haven't seen a complete concise answer anywhere.
The activation problem with Sidekick Slides is common. Most of the time, people buy a Slide without realizing that you have to purchase a "sidekick plan" from T-Mobile to get it fully activated. These phones were build especially for T-Mobile, so they're designed so that you have to have a Sidekick rate plan first to get the phone activated. By the way, this applies to a new phone as well as a phone that's been activated before, but has been 'hard reset" (where you hold the R button while re-booting.. My advice, DON'T DO THIS! Because you have to re-activate the phone.
However, there's good news for all you non-T-mobile folks who bought a Slide on eBay (or from a friend etc.) only to find it needs activation. Here's the solution. If you search eBay for "sidekick activation" you can find folks selling prepaid activation kits for around $15. They include a sim card and about $10 worth of sidekick prepaid services. Buy the kit, install the sim, and go online to register your phone (registering by phone is a pain with T-Mobile). Online you have 3 plan choices: Pay By The Day, Pay As You Go, and Sidekick. Pick Sidekick, and then provide the requested registration information. When done, your phone should be ready to go and should activate within about 5 - 10 minutes. You'll be charged $1 a day and airtime for calls, so it's not cheap, but at least the phone won't go to the activation screen when you startup.
The next question is, how to unlock the phone for other networks? IF it's not unlocked you will be prompted for special code (this is different than the user-defined password that simply locks the phone against unauthorized users). It won't work on other networks until it's truly unlocked. Again, the trusty folks at eBay can help. I searched for Sidekick Slide Unlock Codes and found numerous vendors, ranging from $6 to $12, who provide permanent unlock codes for Slides. I found on that provides the code via email (others want to mail it to you) and got my code within 30 minutes. So now I can remove the T-mobile sim and put my ATT (or other) sim in and go. I should mention that some of the Sidekick features ONLY work if you have a sidekick plan, but that's T-Mobile's and Danger's devious attempt to make us miserable. Good Luck! Hope this helps! Rick

Menu on it's own doesn't work - it will take you to the "emergency call" screen. You need to press "menu" + "L" to bypass the activation screen.

you have to go to and authorized t mobile retailer to register your phone or to activate it.

I will give it to you for free. Push Menu + L to bypass the waiting for activation screen. You will need to do this every time you restart the phone.

If it is stuck at the activation screen it hasnt been unlocked. The Iphone requires a 2 year agreement to even use the phone. So if you thought you were buying an unlocked version you were conned an unlocked and activated Iphone wouldn't even show an activation screen. If you want to read up on how to unlock and activate your Iphone I suggest readding the helpful guides and free help located here.

Clicky

the phone is stolen and has been canceled if u cant activate it

I had the same problem when I bought my unlocked Sidekick.  Hold down the Menu and "L" buttons for about 5 seconds until the "waiting for activation" message goes away.

press menu and the letter L at the same time so u dont have to wait 2 days of no phone using

Call ATT and have them activate those features in that phone.

Geno