Hello im using kamailio and rtpengine to build a custom SBC.

i was using freeswitch as my b2bua but i noticed a lot of issues when cps and simultaneous increased so i tried sems and i have better results.

But i do not really understand sems license so i want to know if i can use it.

i read from sems github

"It is licensed under dual 
 license terms, the GPL (v2+) and proprietary license. This
 program is released under the GPL with the additional exemption
 that compiling, linking, and/or using OpenSSL is allowed.

 For a license to use SEMS under non-GPL terms, please contact
 FRAFOS GmbH at info@frafos.com"

so my question is if i want to use sems as b2bua with kamailio and rtpengine and make profit of it do i need to purchase a license each time i want to sell it to a customer ? 

Thanks.