[cisco-voip] Problem with PSTN calls

[yy]

Hi,

I having a problem with my VoIP phone making calls to local PSTN number. 
Now, if I make a call from a local VoIP phone to another local VoIP 
phone, we get :

** Without sip phone user 1002 picking up the phone

Sip phone 1001        SER                    Sip phone 1002
192.168.5.6             192.168.1.2        192.168.4.4

  |    Invite                    |                                    |
  |-------------------->|                                    |
  |                                |    Invite                        |   
  |                                |----------------------> |
  |                                |                                    |
  |                                |    Trying 100               |
  |                                |<----------------------|
  |     Trying 100          |                                    |
  |<--------------------|                                    |
  |                                |    Ringing 180              |
  |                                |<----------------------|
  |     Ringing 180         |                                    |
  |<--------------------|                                    |
  |                                 |                                    |


Now, if we make call to a local PSTN pots number again without the 
called party picking up the phone, we get a 200 OK straight away! If I 
understand correctly, I should stay at session 183 until the
user at the other end picks up the phone. What might be the problem ?

Sip phone 1001        SER                    Router with FXO card
192.168.5.6             192.168.1.2        192.168.1.1

  |    Invite                     |                                  |
  |--------------------> |                                  |
  |                                 |                                  |
  |    Trying 100            |                                   |
  |<--------------------|                                   |
  |                                |                                   |
  |                                |    Invite                       |
  |                                |---------------------->|
  |                                |                                    |
  |                                |    Trying 100               |
  |                                |<----------------------|
  |     Trying 100           |                                   |
  |<--------------------|                                   |
  |                                |    Session 183             |
  |                                |<----------------------|
  |         Session 183     |                                   |
  |<--------------------|                                   |

Why this ??
  |                                |                                   |
  |                                |    200 OK                  |
  |                                |<----------------------|
  |     200 OK              |                                   |
  |<--------------------|                                   |
  |                                 |                                  |
  |    ACK                    |                                  |
  |-------------------->|                                  |
  |                                |    ACK                      |   
  |                                |---------------------->|
  |                                |                                   |
  |                                |                                    |


Regards,
YY