Well there is your problem.
If you have 64k line without compression for voice than as soon you start a call (64K) your data transmission will be cut our the other way around.
At least you need 128k and Q&S on your router.
The only i know of who can do this is the Cisco 1751V or if you have a vpn wan connection to a high-end Cisco voice enabled router you could use a 836 or 837 in stealt modus