We have a CUCM 12.5 system with CER and a SIP trunk to a CUBE router and then on to the local cable company.
We have an issue where an outside caller at number 111-111-1111 calls a phone on CUCM with DN 222-222-2222 that's forwarded to cell phone number XXX-XXX-XXXX. The call does not go through if the XXXX number is a Verizon cell phone (dead air for several seconds then call terminated). The call does go through if the XXXX is a T-Mobile or AT&T cell phone.
We tried enabling Redirecting Diversion Header Delivery - Outbound on the SIP trunk with no luck.
We then changed, on the CUCM SIP Trunk config, under Outbound Calls, the Calling Party selection from "Originator" to "Last Redirect Number (External)". With this setting all forwarded calls go through, including to Verizon. SO that part is good.
However, changing to "Last Redirect Number (External)" breaks 911 calls through CER. Those calls now get a fast busy and SIP debugs show the 911 calls as coming from 911@x.x.x.x in the SIP header, not the ELIN number.
It seems strange to me that only on calls forwarded to a Verizon cell phone do we see this issue. Would modifying the diversion header help? I've seen a few posts showing different diversion header mods that may or may not work. I would only want to change anything on the calls forwarded to the specific Verizon number.
Any ideas how to fix the call forwarding without breaking 911 calls?