One of the things I have observed in the past when offering Internet access at meetings is that ISPs return IP addresses of DNS servers which are only slowly or unreliably answer DNS requests. This is why by default I don't go to these anymore but use Google's own DNS servers reachable via the easily memorizable IP address 18.104.22.168. This IP address is reachable in all parts of the world I've been so far with only minimal latency.
Recently, however, when I was on the Philippines, I noticed that while the Google DNS server responded to ping requests, there were no answers to DNS requests. As the issue continued over a number of days it seems that either the outgoing DNS requests were blocked or incoming DNS responses were discarded before reaching me. I did a search on the web to see if there is any information whether the Philippines block foreign DNS servers but came up empty handed. If you know more, please let me know, that would be quite interesting.