On higher spec machines, you can maintain sound quality while increasing the number of concurrent sessions. Multi-CPU machines are particularly effective.
Concerning memory capacity, memory swapping is known to slow performance and reduce sound quality.
At least 512 MB of RAM is required to run PBX. 1 GB or more is recommended.
An effective way to optimize performance is to pass parameters to the Java VM to increase the memory used by PBX or SIP Server from field [Java VM arguments] at PBX > [Options] or SIP server > [Configuration]. Set as “-Xmx256m” (means 256 MB). Default setting is 64 MB