libamsip is owned by ANTISIP SARL (COPYING). PLEASE CONTACT <email@example.com> FOR COMMERCIAL LICENSE.
The eXtended eXosip library is based on osip and implements a lot more SIP stuff so you can quickly build compliant SIP applications. The API is enough flexible to allow you to control and complete SIP messages. Thus, it keeps the stack extensible.
While osip can be used for any SIP applications, eXosip2 only provide APIs to control endpoints.
amsip is a COMMERCIAL LIBRARY which provide a complete and simple API for softphone. It adds to osip/eXosip RTP support, codec support (speex/ilbc/g711a/g711u/...), and audio support on linux and windows.
For any use of eXosip2 beyond the rights granted to you by the GPL license, please contact my company at <firstname.lastname@example.org>.
With oSIP (which is LGPL), you still have the choice to implement propriatory application and link with oSIP. For the benefit of the community (and for your own benefit), please consider implementing your application in GPL.
Still with osip, you have to respect the license terms. Mainly, users of your software must be able to fix bugs in osip themselves. (they must have the source code, be able to compile it and link your product with the new version.)