I use a US Robotics 33.6 sportster voice external modem. They are older but plenty fast enough and simple to set up. You can try a search on ebay.
I picked up a few as a package deal on ebay a while ago and I have an extra. They were only a few dollars each. Sent me a private message with an address and I will sent you one as my contribution.
You will need to enter an init string of "ATE0" (that's a zero) into the camera. This turns off the echo feature, the camera will abort the connection if it gets an echo from the modem.
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