You just have to simply wait until you are in a area with signal before using your phone again. This is a highly debatable topic but the author of this FAQ recommends that you follow and take all warnings seriously until proven otherwise. The IMEI number of your phone is the serial number located on the back of the handset. It is the same as ESN numbers on the older analogue phones. To be able to use your handset and SIM card overseas the network has to support it.
Some countries allow automatic roaming and others allow plastic roaming.
A phone designed for the GSM900 system will not work in countries that support only DCS1800 or PCS1900. These phones are expected around 1998. Plastic roaming is when you take your phone with a special SIM card from your service provider or from a service provider in the foreign country. With automatic roaming you get to take your phone and number overseas. All you have to do once you have arrived at your destination is switch your phone on.