Squeekz,
I'll leave it up to you to decide if you just want to send in your voucher or whether you want to try once more via the phone just to confirm the address. However if you do call, first make sure that once you reach an agent, that you then ask for customer service. Agent's can't deal with this issue and if you aren't asking the agent for Customer Service, then you aren't speaking with the correct people.
Once you have a customer service rep on the phone, if they once again tell you no, then ask them to pick up the latest National Timetable and flip to page 125. Halfway down the left column on that page is a section entitled "Time Limits". In that paragraph is specifically states "An exchange voucher is valid for one year from date of issue, and may be renewed provided this is done before its expiration date." Ask them to explain how they are correct in light of that.
You may however just find it easier to run down to your local post office and just send the voucher in by next day air service, along with a note asking that they reissue you a new voucher. If you do that, to the best of my knowledge it still goes to the revenue center in Philly. This is the same place that one can mail an unused ticket in for a refund, so it makes sense that they deal with the vouchers too. But note that the timetable doesn't actually say that this is where to send the voucher to, although I seem to recall that this is where they were sent in the past. That address is:
Amtrak Customer Refunds, Box 70, 30th St. Station, 2955 Market Street, Philadelphia, Pa. 19104-2898.
By the way, I highly recommend using the US Postal service as Fed-ex and other services like that sometimes have issues when trying to deliver to a post box. The US Postal service does not.
Good luck!