May be a long shot but verify that PHONE RELATED INFO/FLAGS/MANUAL FORWARD TO PUBLIC NETWORK is on for each phantom.
If that's not it I'd replace one of the phantoms with a real phone or SL port and forward that, then test. Object is to see if it's just a problem with your phantoms or true for any device. Phantoms had some interesting behaviors in early software versions, especially regarding forwarding, since they were treated as unplugged in some versions.
Also, possible the TRANSFER-VOICE PROCESSOR might be coming into play here? Try setting it way out, see if that fixes it.
Check the PROPAGATE ORIGINAL CALLER ID ON TRANSFERS...with some carriers they won't accept outbound caller ID you don't own and drop the call, especially if it's LD.
All just guesses, this is actually pretty common practice and usually works