The 1531P is a gigabit switch, layer 3 lite. It does no SIP processing, it simply forwards packets. According to the spec sheet it is non-blocking with a theoretical throughput of 24 Gbits/s assuming all 12 ports are forwarding to each other at wire speed.
At roughly 80 Kbps/s per voice RTP stream on a gigabit link, that would be 12,500 simultaneous voice calls per port. Less for video.
In other words, it will handle as many voice calls as any gigabit switch is capable of. Its limitation is the physical port speed.
Thank you for the clarification.