Rare situation in SPA942 or asterisk?

I have a trunk for outbound calls to USA, i have a SPA942 of Linksys and i have an analog port “FXS” in a TDM400P
so, everything it’s fine and running well, but when i sometimes call to usa VIA the SIP Trunk using the analog phone via FXS port, the call progress fine, but when i call using the SPA942 phone, just sometimes i can listen the tone calling to USA, but when i call using the analog, everything it’s fine…
and i have configured the outbound proxy correctly in my SPA942 to my asterisk server.

I don’t know… as i said, just sometimes i hear the ringgin tone to USA using the SPA942
Maybe problem in other outbound config somewhere?