Digital systems are unable to process analog signals, making it impossible for modems to communicate. Unfortunately, many large hotels, offices and universities have installed digital phone systems. The best way is with a digital line tester. There could be a special connector for a data port, but you must find out if this is digital or analog. Plugging into a digital system can potentially fry your modem, because the voltage level is higher than what many modems can handle.
Line noise is another problem, especially in many less industrialized countries.
In most cases, you are at the mercy of the telephone exchange but there are a few tricks that might help reduce the interference. First, try connecting at lower modem speeds. Try reducing to lower speeds a step at a time until you get reliable results. Or, you can try working very late at night when there are fewer people using the network. There is also a type of deliberately created line noise called tax impulsing.