With LGPL you only need share what you modify, with GPL you need share all ‘derivative work’. The test is fairly simple, if your application can compile only in the presence of opensource binaries or source files, it must be shared.
You do not have to send the changes to the community. But if you don’t, when someone request you must give the source code to that person. If will not do so then you loose the right to use and distribute the GPL code.
To Teodor Georgiev:
Vocal is not GPLed.
However PortaOne uses a modified SER as the heart of PortaSIP. SER is GPLed and so is Asterisk and GnuGK.