Alti-IP 600 Phone freezes Occasionally

Our LAN is connected to the Internet usina 512Kbps DSL Line. The DSL connection is given to the WAN port of a LINKSYS Router. This LinkSys Router is directly connected to a COMPEX 8-Port Switch, to which One Alti-IP 600 IP Phone and 5 PCs are connected. This Switch is Up-linked to a 16-Port Dell Switch. The Dell switch has 3 IP Phones (all Alti-600) and 4 PCs connected to it. When outside calls come, occasionally the ststus of the Alti Ip Phone changes to BASIC-Idle. Now we canot receive or initiate calls.
After rebooting the Linksys router, everything becomes OK.
Could someone please help?