Thanks for your reply Jamie ! phones are registered to cucm which is at DC. for pstn one e1 is terminated at local GW at the site. if we reach via pstn then there is no issue and calls are forwarded properly. Issue is only while reaching extension over the WAN. And I have not compared any logs as frankly I am not aware of which debugs to compare.
My gut feeling is, this is CSS related, and more precisely, your call forward CSS's.
make sure that these call forward CSS's can call mobile numbers.
quickest way to test is to use Dialed Number analyzer, or stick a test phone's line into a call forward CSS and call a mobile number and see if it established the call
if you are using 0 or 9 for an "outside line" do you have that in the CFWD All number as well?
also, what gateway are you using to make that forwarded call to a mobile? is it possible to send the debugs of a failing call, like ccsip message, debug dial-peer.