If this is an 1800 with an 8 port managed switch on it you would apply the ACL onto the "interface VLAN 1" using "ip access-group 101 in" or you apply it to the outside interface or WAN port it will ussually have an interface number like fastethernet 0/0.
Now as far as excluding the VOIP box even if you are using the router for DHCP you can do a single "reservation" per DHCP pool so to speak and just exclude that address from your normal address range. So if your mac address of the VOIP box is 1111.2222.3333 and you wanted it to always be IP 172.16.2.254 address the commands would be.
ip dhcp excluded-address 172.16.2.254
ip dhcp pool Data
host 172.16.2.254
hardware-address 1111.2222.3333 ieee802
client-name VOIP
You could then add this to your ACL
access-list 101 permit ip host 172.16.2.254 any
access-list 101 deny tcp any any time-range 4pm6pm