Hi Sariaga!
Using sip servlet or any other method you would have to have the same syntax in you too and from. It also depends how your VoIP provider has set up his backbone and SIP PSTN GW.
First of all the from should represent your SIP URI it could be the username concatenated with the domain or something different. You could use a network analyzer like wireshark and look at the register you are normally sending. In the register both to and from will contain your identity.
That one you should use in from header.
Then I guess that in the To header you should use the same except you replace the user part that represents you number with the number of the person you are trying to call.
In some cases you would be able to reach the PSTN GW but most operators would only allow the proxy to access it because in other case they will do the charging and the identity check with the help of 407 proxy authentication.
But if the call is between two users within SIP domain and no PSTN GW then it is only
From: Sariaga <sip:0560xxxxxxx_at_bmnhb-02.bt.com>;tag=12345
To: Somefriend <sip0560yyyyyy_at_bmnhb-02.bt.com>
BR Kristoffer
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