Unless you find a way to change the Cisco modem/router's mac address you won't get a new ip address unless Melita randomly decides to change ips, but Melita's Ip's do not change very often unless you change the mac address. Altough if you change the mac address of the cisco modem you might end up not getting any internet at all. I'd use proxies as MercyKiller101 said
If you do change the CISCO Modem MAC Address (which is impossible) you will have to call melita to re-register your modem on their system.
Call Melita and ask them to refresh you IP address, invent some excuse along the lines of "my webserver needs an IP reset as it's being blocked" and they will grant you an IP refresh. The refersh usually takes 24 Hours to work, unless you call at the exact time Melita is refreshing their servers, which in that case your IP will change immidiatly. Also after the 24 passes, restart your modem to get the new IP parameters...