I had the same issue with my 8220. The problem was that the card was beat up so bad from being pulled in & out so much I had pretty much worn though the contacts. I'd frequently have to power off readjust the card then power back up, usually solved the problem for me.
Sorry to tell you this, but the 8520 is an EDGE (2G)-only phone. Bell doesn't have an EDGE (2G) network, only an HSPA (3G) network, so you'd need a Curve 3G (9300), Pearl 3G (9100), Bold 9000, 9700 or 9780 or a Torch.
Either those or a phone on their older CDMA network. You'll notice they offer the 8530, which is the 8520's CDMA sibling.
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