Not necessarily
After having a spate of DNS problems with my ISP, they Tech gave me two DNS server addresses to use: One was theirs, and another was a "secret" address of a DNS they use when theirs go down. (Neither were able to resolve "z.iwethery.org" for about a week after the move). So simply having a single ISP does not necessarily imply a single point of (DNS) failure, unless its hosts all yourr accessible DNSs.
jb4
shrub\ufffdbish (Am., from shrub + rubbish, after the derisive name for America's 43 president; 2003) n. 1. a form of nonsensical political doubletalk wherein the speaker attempts to defend the indefensible by lying, obfuscation, or otherwise misstating the facts; GIBBERISH. 2. any of a collection of utterances from America's putative 43rd president. cf. BULLSHIT