I have one client that has the google dns servers set in his router 8.8.8.8 and 8.8.4.4. This makes all his voip phones use the googles servers for DNS of course.
I have another client that that uses his ISP’s dns at router level. Therefore his voip phones show his gateway as his dns 192.168.1.1 - know this is just referring the dns query back to the ISP.
Which way is better? Any pros or cons to either way?
Never use ISP DNS ever.
Perosnally, I recommend using Cloudflare -> Google -> OpenDNS
Cloudflare: 1.1.1.1 / 1.0.0.1
Google: 8.8.8.8 / 8.8.4.4
OpenDNS: 208.67.222.222 / 208.67.220.220
Now to answer this question, you have to realize that you actually did not ask what you thought you asked.
It is not Google DNS versus ISP DNS. What you described is Google DNS versus router DNS.
I, 100% of the time, always want the router (or local AD DNS or Pi-Hole) to be the DNS that is in all of the hardware within the LAN.
The DNS server (router, AD, Pi-Hole) should be the only device that reaches out to the public DNS servers. In fact, I block outbound DNS traffic on all corporate networks that I have control over except for the DNS server.
You are absolutely correct. It is Google DNS vs router DNS. You made perfect sense. I would put the dns entries at router level - not phone level - that way when you look in the GUI of the phone you would see the router IP address as the dns server. Then I would put cloudflare (example) in the router level.
Thanks
It doesn’t really matter, if you use your local router as your DNS (no matter whether that is set to use 1.1.1.1 or 8.8.8.8 or 4.2.2.1 upline, or your VSP’s DNS resolver) there will likely only ever be a handful of VOIP relevant addresses in it’s cache, If you are concerned about often changing A records (DDNS for example, then a far more relevant metric is the TTL of the record, Google is a lot quicker than CloudFlare there , but your local router or indeed DNSMasq on your machine can be better tuned if needed.
In what reality?
In this context, honoring TTL and A records dynamically changing
Definitely the opposite of my experience.
For performance, reliability, control, and possibly for putting in your own local records, I’ve found it best to use a local DNS server - whether that be your router, AD, or I spin something up under UNIX. The local caching speeds performance (slightly). In other words, don’t point the individual phones (or anything else) at an external server such as Google.
In turn, your local DNS server will reference an external DNS server - which might be Google, Cloudflare, your ISP. The quality and speed among ISPs varies (which you might think odd, since the ISP is right at the end of your wire). Who knows what Google logs and how long they retain it - their speed is solid, their reliability high. Cloudflare tests a bit faster than Google, and they keep a log of requests for 24 hours only (in case they have a problem, and they need log files for problem determination). I use Cloudflare.
Indeed 
. . .but your local router or indeed DNSMasq on your machine can be better tuned if needed… . . .
Don’t even think of using your ISP’s DNS server for VoIP applications, directly or otherwise. When a DNS lookup fails, they generally redirect to an ad server or ad-supported search engine.
So, if a SIP server lookup fails (you fat fingered the domain name, routing problem on the internet, someone forgot to pay their DNS bill), the device or PBX starts sending REGISTER requests to the ad server. Of course there is no reply, so you assume that this is a firewall or routing issue, wasting time looking in all the wrong places.
If you have the patience and the knowhow, seeding /etc/hosts with all your providers and external extensions precludes the necessity of any DNS provider.
That is the first place linux looks, if it gets an answer the DNS query is never made. So it is without doubt the quickest “resolver”
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