Verizon has by far the best service. If I forward my Verizon Eris number to my T-Mobile Blackberry, are my calls eminating from Verizon or T-Mo? In other words, when I'm dialing from my blackberry am I getting Verizon service?
No. Your BB is using T-Mobile service. Keep in mind when you forward calls between phones/service from different carriers, you are using up minutes on both. You are using up your allotment from Verizon for the forwarded calls and using up your T-Mobile minutes as well for it receiving calls. It's not as simple as it seems to use call forwarding this way. Only calls made and received by the Eris are on Verizon's network.
You actually burn 2 minutes for every minute on the number being forwarded from: one for the incoming call to that line and another for the outgoing call to the forwarded number. So, factoring in the call on the phone receiving the forwarded call, and that's three minutes used for every actual minute of "talk" time. Pretty pricey in my book.
And why it's important for customers to ask carriers how Call Forwarding works between them and how it's being charged, otherwise you'll get an unpleasant surprise when the bills arrive.