RJ 11 - RJ 45 converters?

Ive recently got a Cisco E4200 router, My ISP is Srilankan Telecom bases through telephone lines, I before had a Prolink router which i connected a RJ 11 cable ( normal telephone cable) to the router to connect to the ISP, The new cisco router has an RJ 45 slot, ive consulted the CISCO support and they told me i had to use a modem to connect to the internet. Is there a way that i could convert the RJ-11 to a RJ-45 using some jack? so that i could directly plug it into the Cisco router?