ISPs have covered the planet, so it's almost always just a local call charge to get the data onto the internet for access anywhere in the world. Cellular modems can connect systems in locations not supported with land phone lines, albeit at a slower speed and higher cost than conventional modems. 56K modems offer faster speeds on downlink. How about cellular access. If the site has cellular, and slow frame rates are acceptable, cellular is a good choice.
Occasional access is the best option, since cellular access charges by the minute, and is expensive.
Other link options, including spread spectrum radio, have higher installation costs, but don't require local cellular availability and are much less expensive to operate with no airtime charges. The options for 'the last mile' are varied and can handle almost any circumstance - if you can afford it. ISDN is still a good choice today if DSL or cable modem access to the internet is not available. Either or both lines can be used for data or voice phone calls. In effect, you get 128K of data bandwidth at a pretty reasonable price.