Thanks to all in advance for helping me out, and sorry if this seems complicated...

My iPhone 3G is stock 3.0 FW, 06.15.00 BB, locked to at&t, proper APN carrier settings installed for my sim card and it is not getting service (it should be). When I put my sim card (at&t mvno) in it chimes (the voicemail sound) and recognizes that I have 3 new voicemails (by showing me a number 3 on my phone app) but then, after a second or so, the circle number disappears and my iPhone does not say "Searching..." like I know it's supposed to. It just shows the "one bar" and no carrier. I know that the voicemail notifications aren't supposed to disappear and so I'm thinking this is why my iPhone isn't getting service with my sim card.

Why would my voicemail notifications disappear and do you think it's got anything to do with my sim not getting service and how would I fix it?