Asterisk 1.8 + spa400
1. From Zoiper I call extension spa400ee, number 411111 is called successfully, across port 4 os spa400 on line 1
    exten => spa400ee,1,Dial(SIP/L4411111@spa400e)
2. Hangup normally
3. Call again, I receive from spa400
       SIP/2.0 503 Service Unavailable
Green Led on Port 4 of spa400 is off but cannot call on this channel;
if call another line, for ex. Line1 with Dial(SIP/L1411111@spa400e), call is successful
4. If reboot spa400, I can call normally one time on each line
On inbound calls issue not happen; I can call and hangup, call and hangup without problems

Any idea ?