As a seasoned Linux Engineer and Full Stack Developer with comprehensive skills in Asterisk PBX, Debian, Linux, and VoIP, I am confident in my ability to resolve your current issue with Freeswich - Astpp. My proficiency spans across Ubuntu, Centos, Kali Linux, Debian, Rocky Linux - names that are symbolic of quality open-source systems configurations and troubleshooting.
In regards to your problem of being unable to establish SIP connections after installing Astpp on Debian 10, I believe this stems from certain specific configurations that could have gone awry during the installation process. As an expert in Asterisk PBX and Debian, I understand the intricate workings of these systems and can quickly identify unlikely sources of the problem.
My previous experiences with similar projects have taught me that vigilance is key. I won't stop until I've thoroughly vetted every possible avenue for issues. Trust my proficient problem-solving ability paired with my dedication to deliver outstanding results within tight deadlines. Let's rectify your current stumbling block together and ensure uninterrupted SIP connections for a seamless operation on Astpp.