Making international calls is different from traditional local calls. To call friends and family in Nigeria, you will need to dial your current country code, then the Nigerian country code, Local area code in Nigeria, and then the phone number you wish to reach.
For example, if you are making a call from the United States to Nigeria and you want to call someone in Nigeria with the phone number 08035555555;
You will dial thus : 011-234-8035555555, where 011 represents the United States country exit code, 234 is the Nigerian Country code and 8035555555 is the number you are trying to reach in Nigeria. Alternatively, you can replace the country exit code (011) with (+), so you can just dial +2348035555555.