Commands suggested by @JiaChao worked for me yesterday, but unexplicably the problem came back after one day, so I shut Mac down. So, I continued searching and found a definitive solution by reading itim dot co article and figured out that Mac must have it own options to /etc/ppp, so I found also in taoofmac site and think the problem can be somewhere the first article said (LOCAL IP) so I commented all uncommented option but noipdefault. It finally works.