X-Lite: some IP calls lose audio ~3secs

[Tslam]

My X-Lite registers to au.voxalot.com, and passes the echo test. It also calls eg 1-516-687-5089 (supernettel.com) and confirms ENUM. Supernettel also works if called with the Sipbroker prefix *013.

But... some IP calls to eg *266300 (monkeys) and *393612 (NY clock) lose audio after a few seconds. Why?

I've configured X-Lite as follows:

. discover global IP address
. Use STUN stun.voxalot.com:3478
. enable ICE *
. manual port range 5070-5080 ; to avoid conflict with my ATAs *
. send SIP Keep Alives *
. contemporary registration with same Vox ID on my ATA port 5060 *

* apparently makes no difference - still registers with same audio symptoms without enabling these options

I haven't explicitly forwarded any ports thru my NAT router, but why should I. Besides, I don't know what RTP range X-Lite is using (that's STUN's problem).