Qnewbie

You should disable message waiting(something like that) in SPA3000 setting.

Qnewbie

There is no easy way to debug.

Disconnect from Asterisk and connect to other ITSP and find if there is any mysterious call signals.

If yes, it should be the problem of your SPA3k. Then you have to set up something in the middle to monitor the traffic from/to SPA3k and find out what's going on.

If no, it is your asterisk problem(I think this is most likely reason). Change your asterisk setting would solve problem.