I wrote this article believing there was no solution to this problem. However, a person in the comments has recommended installing the Opera mini browser as a solution. If you already own a Blackberry, this is definitely worth a shot. You would have to download the broswer's installer file to a computer and then transfer it to your phone and install it. If it works, this would fix only the internet, not MMS.
Even though you can't access the Internet or send and receive MMS, you can still make and receive phone calls and send and receive regular text messages, but since one of the main reasons to own a Blackberry is to use its data features, this can be very frustrating.
Now, I really don't know why it doesn't work. But, I'm going to offer a hypothesis here. So, let's consider a few things...
There Is a Special Data Plan for Blackberry Phones
When you have a postpaid (or contract) rate plan with AT&T, you can purchase a Blackberry data plan. All other PDAs would get the Smartphone Personal data plan. Neither of these plans are available for prepaid, but most PDAs can access the internet on prepaid. But the fact that there is a special data package for Blackberries may indicate that Blackberry phones are not using AT&T as their Internet Service Provider (ISP).
A Different Gateway: Another Clue
If we look at the Internet tutorial page for the Blackberry Bold, we will see that the phone is using a gateway IP of 22.214.171.124. All other AT&T phones appear to use a gateway IP of 066.209.011.032 or 066.209.011.061.This fact would further support the idea that Blackberry phones are not using AT&T for their ISP.
BIS, Blackberry Internet Service
Turns out there is something called BIS, which stands for Blackberry Internet service. If Blackberry devices are somehow using Blackberry as their Internet service provider (ISP), then customers who buy the Blackberry data plan are probably buying access to the Blackberry ISP. Since the Blackberry data plan is not available for prepaid, then prepaid customers are not able to buy access to BIS. Therefore, prepaid customers with Blackberry devices are unable to access the Internet (which would also prevent MMS capabilities). Again, I'm not saying that this IS the case; I'm just saying that this MIGHT be the case.
But Some Blackberries CAN Access the Internet on Prepaid!
Some Blackberry devices do seem able to access the Internet on prepaid service. I would bet dollars to doughnuts that these are devices that once were active on a postpaid rate plan that had a Blackberry data plan. Again, I'm just postulating here, but I bet that the phone's IMEI number, which is kind of like a serial number, has been logged into a database somewhere on Blackberry's servers as an IMEI that is allowed to use Blackberry as an ISP. So, even though the phone is now on a prepaid account, Blackberry's servers still view the phone as "authorized" to access the Internet through its service. If this is the case, one may expect that periodically Blackberry would purge their database of old IMEIs and such a phone may not always be able to access the Internet.
Therefore, if you are thinking of buying a Blackberry and maybe to be on the safe side, you put your prepaid SIM card in it to make sure you can access the Internet, just because it works now doesn't mean it would always work. And, remember that AT&T does not appear to support data access on Blackberry phones using prepaid service. So, if it works when you buy it but stops later, you're out of luck.
But Some Work on Prepaid Right Out of the Box
It's true. I have seen some Blackberry model -- I can't remember which one, but I think it was the Curve -- that was able to access the Internet through prepaid right out of the box. I'm not sure if they're always able to access the Internet or if that's just a temporary state that the phones are in right after purchase. Again, since AT&T's official stance appears to be a Blackberry devices are not compatible with prepaid, it is best not to purchase a Blackberry device at all if you intend on using it on AT&T's prepaid service...unless AT&T changes their official stance on this issue, even if only with respect to certain models.
So, if you have a Blackberry phone and you're using AT&T's prepaid service and if you're not able to access the Internet...I hate it for you. If you're an AT&T prepaid customer whose thinking about buying a Blackberry phone, I hope I've convinced you not to. There are plenty of other PDAs that CAN access the internet on AT&T's prepaid service (my Samsung Blackjack II, for example). But, AT&T's prepaid internet has always been a little wonky, so whatever model you're considering -- even if it's not a Blackberry, you should always research it to make sure it can access the internet on AT&T's prepaid service.