BSLillo

Hello everyone!
I have a problem...i'm trying to build, programming in VB, a sort of web
phone client in order to make sip calls...but my problem is that when I try
to make a call from a pc on which is installed this web phone client, with
an external public ip address, to an ip Phone which is behind a Nat through
a Server Proxy,...i can't establish in the source code a prefixed port like
the 5060 to establish the sip connection...infact, sometimes it occupies a
port which is open on the firewall, and somethimes no and in this case i
can't let communicate the two phones!

I saw for example the log file of X-Lite Client, which goes very well and
all the sip communication messages goes through the same port, the 5060, and
in this case it's all ok because i've opened that port on the firewall.

I saw the RTCClientPortMapping but i'm not able using it.... if someone could
help me i would be very gratefull..
some source example code....in which way could i set definitively the use of
port 5060 for sip connection (Invite,Ringing,ACK..)?
Some examples..i saw on the RCT Client API documentation, but there are no examples of how doing this port mapping...i'm not able to create the source code, if anyone have already done it and want to help me, please!
Please help me!!!
Thank you very much in advance!!