Reza
The PARADYNE 7975 M/SDSL is not a CSU/DSU device which would be used on a traditional serial circuit. This unit is for termination of Symmetric Digital Subscirber Line. Which kind of provider circuit are you connecting to?
Depending on what is in the provider equipment connecting to both ends it could be correct to correct connect Router 2621/2610XM to 7975M/SDSL and the same at remote site to implement Frame Relay. (if the provider has DSLAM and Frame Relay switch providing that circuit) The manual for the 7975 shows connecting to a router with Frame Relay encapsulation as a supported configuration. See this link for additional details:
http://www.optimumdata.com/shop/files/paradyne/dsl-mvl/7975-A2-GB20-40.pdf
HTH
Rick
HTH
Rick