Switching to ISDN could bring your telephone bill down by 70%, if you are able to find the best service. You will search ISDN line rental packages and choose the most affordable package. It could be a mistake as you could pay hidden charges with the monthly rental.
Every VoIP company has some packages to offer. Every company has some services to offer. Every company has something different to offer. You should know what the companies are offering. Get their ISDN line rental packages and compare the packages to know what the companies are offering. It is the only way of finding the best VoIP service.
Choose the package that includes the services that you need. Look at the services instead of monthly charges. It is services that will save you money. For instance take the package with unlimited calls. If you make more national and local calls then you should choose the package with unlimited calls. See the ISDN line rental packages of all the service providers so that you have an idea on what the VoIP companies are charging.