Your problem is the unescaped spaces in the URL. This, for instance, works:
NSURL *url = [NSURL URLWithString:@"tel://1234567890x101"];
Edit: As does this..
NSURL *url2 = [NSURL URLWithString:[@"tel://1234567890 ext. 101"
stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding]];