ESTABLISHING A VPN THROUGH AN ISP

MY COMPANY IS GOING FOR A VPN CONNECTION THROUGH AN ISP LEASE LINE , I WILL BE GETTING ONLY ONE LEASE LINE FROM THE ISP ,USING A LOCAL TELPHONE LINE I NEED TO CONNECT TO MY OTHER BRANCHS ALSO. COULD YOU PLEASE GIVE ME A SOLUTION HOW DO I ESTABLISH THIS CONNECTION ,DO I HAVE TO WRITE SOME PROTOCOLS ? I WILL BE USING A CISCO ROUTER MODEL 2600 SERIES.
PLEASE SUGGEST .

THANKS & REGARDS