Simul Ring to Local SIP and remote IAX Doesn't Answer

I attempt to do (on Asterisk 11.5.1) a simultaneous ring with the following dial string:

dial(SIP/4401&IAX2/priv:redAnzK+ezXYZfsSvmKBZA==@192.168.0.1/278,30)

Both devices ring properly. If the SIP/4401 device is picked up the call proceeds as normal. BUT, if the IAX/278 device picks up the call never connects, 4401 continues to ring, the caller hears the ring continue, the IAX/278 device stops ringing when the line is picked up but there is no audio (or dialtone). The 278 is a SIP device on the other end of the IAX inter-Asterisk connection. Simply doing dial(IAX2/priv:redAnzK+ezXYZfsSvmKBZA==@192.168.0.1/278,30) works fine.

If it matters this dial is performed inside a GoSub.

Any suggestions or comments?