Where to apply in- and out-manipulation-id?

Hello,

If I have a call like this:

Provider —> SBC_Provider_Realm —> SBC_Customer_Realm —> Customer
This means my Ingress realm is SBC_Provider_Realm, and my Egress Realm is SBC_Customer_Realm.

I then receive a REFER from the Customer, which should be forwarded (as a REFER) to the provider.
But, I want to apply a manipulation on the Refer-To header before the REFER is routed, and also a manipulation before the REFER is sent to the Provider (after routing), on which Realms and in what direction (out / in) do I need to apply those two rules?

Thanks