shayan,
You cannot change the signaling port on the A800.
You mention good pdd vs low pdd, I think you are confused on terms. PDD is Post Dial Duration. This is the time between the user dialing the last digit and getting a ring back. The longer this duration, the more likely the user will hangup because they think that the connection is not going thru.
You probably are talking about duration in that you are having low duration and there could be many reasons for this, like dialing wrong numbers.