Dialing from PAP2 polarity reverses before ring

I have a PAP2 configured with an Asterisk extension; when I dial any number, the line on the PAP2 is polarity-reversed even “before” the called party is ringing.

Any idea? Thanks

What software version are you running on PAP2?
I have PAP2 with the latest version and have no problems with Asterisk (1.4).