Some services will automatically cancel your free fax service if no faxes are received for 30 days. How it works is that you sign up online for the fax facility and they give you a phone number which you can then distribute. When someone faxes that number, a TIF picture file is e-mailed to you as a file attachment which you can then open and print. In the Internet ISP ads of major newspapers and in computer magazines. If the ads and any other restrictions don't bother you, then there are no major problems.
Some services have a monthly maximum of online hours and start billing after that.
Others only let you access certain sites that they have signed up to their own affiliate program. Make sure you have unlimited Web access and keep an eye on the hours clocked up. Free ISPs are becoming increasingly common now so there is the possibility of having more than one at any given time. By having accounts with several free ISPs, you can change to a different one when you are nearing your limit on the first service - that keeps your access free. Usually their programs run for specified periods and have certain intake dates.