SIP PSTN API To Determine Call Forwarding Number on Mobile

I have an iOS and Android app that allows users to set up a custom voicemail service. To enable the voicemail service, we identify the user's phone carrier (Verizion, T-Mobile, AT&T, etc.) and have the user set up conditional call forwarding to a phone number we manage on [login to view URL] Once a user sets up the conditional call forwarding, I need to verify if the call forwarding was in fact set up correctly. I have two options to do this: One option is to call the phone number of the user, have our app reject the call, and then look for an incoming forwarded call from that user's phone number. However, it seems like there could be a simpler process. The second option I am trying to figure out is a way that I can just call the user (it's okay if the user's phone rings once or twice), and then retrieve the actual SIP header information and extract the call forwarding phone number. This way we can see at the SIP / PSTN level the number that is configured for the conditional call forwarding. To apply let me know if you know how to do this, and how long it would take you to set up. welcome candidates.