Maybe in your case, you call into AT&T and ask them to separate your DSL from your phone service telling them you want to keep your email accounts.
PS. ISP specific email address is not a good idea. Gmail or Live mail much better; you are fixed to one ISP.
Line 1 - Home number/w DSL (Elite speed - 6Mb/768Kb)
Line 2 - Office Number
I started the porting process on 7/26 with this configuration. That same day I placed an order to have AT&T disconnect my DSL service on line 1 and ordering new DSL service on Line 2, effectively moving the DSL service off the porting number. I asked how we would be able to retain our email addresses and was told, and subsequently found to be true, that when you register the new DSL service, you will be asked if you have an existing email address with AT&T. I re-registered with my email address and all is fine with DSL working on line 2 since last Thursday.
If you're asking why I kept my office number, the answer is interesting, at least to me. The previous month I dropped an international calling plan I no longer needed and was offered my current speed DSL for $24.95/mpnth if I made a 1 year commitment. I agreed but was surprised that my next phone bill did not reflect the new contract.
I spoke with the Retention Dept about not getting the better rate and they told me that they had the authority to give me that rate. By this time I had decided to go with Ooma, so we discussed my options. As long as I kept a phone line they could give me the $24.95 contract rate, but a dry loop DSL line would cost $40/month. I converted my office line to a measured line ($8.87/month + taxes) and that's cheaper than a dry loop DSL line.
So there's the scoop, you can retain your email address and you may be able to get a better rate on your DSL to boot!