How can I make a phone call in Objective-C?
well if you are talking about using objective-c to make a phone call on the iphone, then you can do something like this:
NSURL *phoneNumber = [[NSURL alloc] initWithString: @"tel:867-5309"];
[[UIApplication sharedApplication] openURL: phoneNumber];
If you are talking about doing this on a mac ,well, then like others have mentioned that is specific based on number of things like, if you are using voip, a modem, connecting through something like an Asterisks box, etc..
This is clipped from a project I did to do just that:
NSString *phoneStr = [NSString stringWithFormat:@"tel:%@",phone_number];
NSURL *phoneURL = [NSURL URLWithString:phoneStr];
[[UIApplication sharedApplication] openURL:phoneURL];