There are a few possible reasons for this.
a) Occasionally, the local ISP may change or delete the phone number.
Always update your phonebook before your next trip.
b) You may encounter difficulties dialing in if several users are trying
to log in at the same time. Try dialing another available number or the next
nearest city number, or wait a few minutes before trying again.
c) If you can connect successfully to another number, it may also be due to
the initial access number or site being temporarily unavailable.
d) If you are unable to connect to the other access numbers, this may indicate
that your modem is not well connected to the phone line or that you have a modem
compatibility problem. Consult technical support or your modem manufacturer.
e) Many hotels require that you to dial a number to get an outside line. Make sure
that you are dialing the required prefixes and local dial code for the region.