Refer rejected by ITSP anyway to make asterisk join calls?

I have calls coming in from an ITSP they don’t support REFER
The call hits asterisk and then by dialplan goes out to an endpoint which answers the call. The endpoint plays some IVR then sends back a REFER and target number to transfer the call blindly. The destination is an external number and fits into the asterisk dialplan.

Asterisk sends back the REFER to the ITSP who give back a 501 and the calls drops. Rather than asterisk passing the REFER back to the ITSP is there anyway to make asterisk place the second call and for them to be joined within asterisk rather than it passing the REFER back to the ITSP?

Or am I confused?