Polycom RMX 1500 Administrator's Manual page 726

Polycom RMX 1500/2000/4000 Getting Started Guide
19-72
Port Gauges
Audio (Voice) resources are as displayed as in previous versions while all
Video resource types are shown as a single group of Video resources.
The gauges show 0 of the 18 Audio (Voice) resources as occupied. The
4 occupied SD resources are shown as 4 occupied resources out of the
total of 47 Video resources.
ISDN/PSTN
The RMX 1500 supports one ISDN card with 4 E1/T1 PRI lines.
On the RMX 2000/4000 a maximum of two RTM ISDN cards are
supported, each providing connection for up to either 7 E1 or 9 T1 PRI
lines.
On RMX 1500/2000/4000, E1 and T1 connections cannot be used
simultaneously.