One VOIP line only requires about 300K. 1.8 Mbps is more than enough.
I doubt it.
VoIP requires 100 Kbps at most.
https://www.cisco.com/c/en/us/support/docs/voice/voice-quality/7934-bwidth-consume.html
At 64 Kbps for a proper codex you get crystal clear sound. Add a little extra overhead for Ethernet, DSL or Docsis overhead, plus IP and UDP, and you'll always stay under 100 Kbps.
So yeah, 1.8 Mbps is more than enough. 500 Kbps would be more than enough too.
What the Op needs to do is configure some form of QoS (priority queuing) on his home router. The thing with bandwidth is: no matter how much you have, you can still always experience congestion. For many applications that doesn't matter much. Especially the applications that do large file-transfers (torrent, usenet, etc). But for delay sensitive applications (like VoIP, gaming, etc) you need to make sure you give absolute priority to that traffic.
My ISP sells VoIP service. (And I have 6/0.8 Mbps DSL). My ISP pre-configures the routers they send out to their customers, so that the VoIP traffic takes precedence over any other traffic. I assume most routers can do that. But if you use your own router you need to configure it yourself. If you use a VoIP-provider that is not your ISP, you probably need to configure something yourself too. (I'm not sure if you can identify VoIP just by looking at port-numbers, or if there is more to it).
The remaining problem is then prioritization in the other direction: from your ISP's network to your home. It's unlikely your ISP wants to do something on their end for you (especially if you don't buy their VoIP-service). So all you can do is try and pray that you are not congesting your download direction. That is where a faster subscription might help. If you have 2 kids watching Netflix, your 6 Mbps down will be full. You can't depend on QoS in that situation, as you'd need QoS on your provider's router. Which, as we just mentioned, you have no access to. The only solution would be to buy more bandwidth than you need. (In the example's case: more bandwidth than 2x Netflix).