The Binding Support Function is used in 4G and 5G networks to allow applications to authenticate against the network, it’s what we use to authenticate for XCAP and for an Entitlement Server.
Rather irritatingly, there are two BSF addresses in use:
If the ISIM is used for bootstrapping the FQDN to use is:
But if the USIM is used for bootstrapping the FQDN is
You can override this by setting the 6FDA EF_GBANL (GBA NAF List) on the USIM or equivalent on the ISIM, however not all devices honour this from my testing.