[SmoothOperator]

When I switched devices from 9700 to 9900, I called the geniuses at AT&T via 611 and announced my intention of doing it. The fellow on the other end told me that I can switch the SIM card without any problem, because as long as I have a Blackberry Data Plan along with unlimited internet on my plan, there is essentially no need to do anything.

Just today I decided to update the IMEI number to the system because I thought that it would be necessary to keep it updated if I decided to unlock the phone. I called 611 again and explained this to the fellow on the other end, who updated my IMEI number without any problem, and gave me the unlock code anyway since I have been a good customer with AT&T since back in the mid-90's when I was a customer with Cellular One (then became AT&T Wireless, then Cingular, and now back to AT&T). He said, in passing, that if I am to fully use the 4G capability, he suggested that I switch SIM cards from my previous 9700 SIM card, which is a 3G SIM card, to a 4G SIM card.

Is this true, guys? Even with my old 3G SIM card in there I frequently see "4G" on the top of the screen when I use my browser, but aside from the superior 9900 browser I honestly do not see much of a difference in downloading speed between 4G and 3G, so maybe the 4G SIM card will make a difference? Also, my concern about updating SIM cards is that the 4G SIM card will have issues with my 9700 in case I decide to put it in there and use my old 9700, so is there concern in this case?

[Howard S.]

You should have the 4G sim in the 9900. No worries about using it in your 9700 if needed as it's backwards compatible.
Posted via BlackBerryForums.com Mobile

[yahjoogooleh]

I have att and switch my sim between 3g/4g phones and I don't notice anything either.
Posted via BlackBerryForums.com Mobile

[snhrph]

I had my old 3G sim card in my 9900. So yesterday, after reading this thread, I called att and was told that in order to use 4G network I need a 4G sim card. I had a 4G sim card at home, so I was able to upgrade.

So far, since the upgrade, I noticed only 1 change - I no longer have an option of choosing 4G/3G or 2G connection mode in Mobile Network menu.

I have not noticed any increase in data speed.

[tsac]

4g speed depends on your location for increased speeds. true 4g speeds offer a lot of speed. I have a 4g/ LTE Wifi Pod from ATT and I average about 8Mb down and 3mb up when in an area offering 4g. LTE is expected to be better.
AT&T Mobile Hotspot Elevate 4G

[dc/dc]

Please remember that none of the services offered in the US are actually "4G" as defined by the ITU.

That said, AT&T provisions SIM cards to allow for access to the HSPA+ network that they call 4G, and without that line code in your account and utilizing the HSPA+ APN you won't be able to take advantage. If you have a 9900, you should get your account provisioned for it. They shouldn't need to change your SIM card for that to happen.