spa2102 with 2 voxalot accounts can't hear voice

[mauromi]

Hi, I've installed a Linksys SPA2102 behind a firewall.
I redirected the followind ports from router configuration to SPA2102:
UDP 2060, UDP 3060 and TCP+UDP 3478.

Then I configured 2 voxalot accounts:
Line1 on UDP2060, Line2 on UDP3060.

Under Voice - SIP - Nat Support Parameters - STUN Server I wrote "stun.voipbuster.com:3478".

Now everything seems working: both accounts can dial external numbers (through dial-out SIP providers like mytcom.it) and can be contacted from external Voxalot users. The voice is clear and the connection never drops.

The problem is that the two phones attached to the SPA2102 cannot talk to each other: when I call one from the other, the phone rings but none of them can hear the other's voice; in other words, they cannot hear each other's voice.

If I use another SIP provider (like utelia or mytcom), it works. It even works if I use one voxalot and another SIP provider. The point is that I need to configure both of them on the same SPA2102 with voxalot.

Thanks for any hints.

mauro