You can use a regular expressions library (like RegexKit, etc), or you could use regular expressions through NSPredicate (a bit more obscure, but doesn't require third-party libraries). That would look something like this:
NSString *phoneNumber = ...;
NSString *phoneRegex = @"[235689][0-9]{6}([0-9]{3})?";
NSPredicate *test = [NSPredicate predicateWithFormat:@"SELF MATCHES %@", phoneRegex];
BOOL matches = [test evaluateWithObject:phoneNumber];
If you're on iPhone, then iOS 4 introduced NSRegularExpression, which would also work. The NSPredicate approach works on both Mac and iPhone (any version).