First you need to set up a route pattern to your remote site. Let s say your DNs on site C are 7xxx and your remote site B is using 6xxx.
set up a route pattern 6xxx and select the trunk Route List in it.
1- Include the Partition of your Route pattern (6xxx) in the CSS of your phones.
2- Ensure that the route list on your RP is correct (ie Trunk Route List)
3- on your remote trunk-> Inbound call -> CSS
check if this css include the Partition of your phones.
You do this just for the C Site, the phones on the B Site will not be able to Call to C Site.
HTH
Zin Elabidine Karzazi