Different P-Asserted Number (PAI) to that of the CID

I use extensions_override_freepbx.conf and the following to set my PAI:

[macro-dialout-trunk-predial-hook]
exten => s,1,NoOp(Adding P-Asserted-Identity)
exten => s,n,SipAddHeader(P-Asserted-Identity: sip:XXXXXXX@blah.com )
exten => s,n(done),MacroExit()

It gets called before the Dial(), so it should be included with every call.