Call 611. Going from one platform to another (like BB to WP) will cause ATT to reset your plan. Voicemail is a feature and needs to be added your account.
My voicemail number in the phone was set to 203 area code (southwestern CT), which I thought was strange, because I am in RI. I changed it to my own number, though it seemed to work fine with the 203 area code number.
I had the same issue. I had to call ATT since the person who programmed the new sim card forgot to re-activate my VM since I had the unlimited data plan. At least that's what the CS told me.