The previous answer - call a cellphone from the landline - is right.
However, if the landline has blocked the feature to call cellphone numbers, then try calling another landline instead, one with a caller ID. Ask your friends who live in the area.
If this is not possible, you can look for a copy of the phone bill. The number should be there.
If you don't have the bill, you can try calling directory assistance. :)
If the number is listed private and not available from directory assistance, call the phone company instead. They should be able to give it to you soon as you answer a few security questions.