have hub(MP5000) and 15 nodes(FXII)

nodes keep losing connection to hub? i can ping each of the nodes IP's but get network busy while trying to intercom or check vm. have to go to the site and press the voip reset button.
hub has 4 voip l. nodes have voip s with 6 channels active at each.
all are connect to a Cisco gigabyte switch.........is that to fast for the voip cards? should i install an old 10/100 switch at each
network used to be connected via point to point but now the network has fiber running to all buildings
nodes are 17F 198.07
help?

Thanks