STRANGE ACTIONS ON CISCO 1700 ROUTER

i use a Cisso 3600 router as our main hub, then i use a cisco 1700 on my remote site. When i connect the routers up using 2 modems and a cross over connection, it works fine. However when i place it on the circut, i will only get line up pro down. But here is where it really get strange! If i change the serial interface on the 1700 and 3600 to 2 diffrent ip address it will come up.
eg: cisco 3600 10.10.1.1 cisco 1700 10.10.1.2 While this configuration is corect i will get line up protocol down
cisco 3600 int S0/0 10.10.1.1 cisco 1700 int s0 10.10.2.1 This configuration is wrong but i will get a line up and protocol up.
Help!