How low you can reduce your costs does depend on your calling pattern. How many minutes per month you call locally; call long distance; call internationally; and how many minutes you receive calls for. If all these figures are low, you can be better off with a pay-per-minute service, if any of the outbound figures are high then you could consider a flat-rate "unlimited" service. So that's the first step, determine your needs: don't pay for what you don't need.
For example, I barely use a "landline" voip phone any more. But it is still useful for incoming calls, and for 911 service. So I use a prepaid pay-per-minute VOIP company, that has a flat rate (< $2/mo) for the incoming number, an add-on fee for the 911 service (80c/mo), and per-minute costs for outgoing calls (1c/min or less to US, Canada, western Europe). I use my own Obitalk box (under $50 to buy). There is a bit of setup and learning needed, but the end result means my "landline" bill is usually under $4/mo (no other taxes or fees).
Example Companies and Forum
Companies in this BYOD (bring your own device) space in the US/Canada include (in alphabetic order), Anveo, Callcentric, Callwithus, Futurenine, Localphone, Voip.ms and many others. A good discussion forum to learn more about them is the VOIP forum at DSLreports (although it can be a bit technical). There is also a reviews section at that site.
If your usage is higher (you make lots of calls to a variety of numbers), most of these companies, and others, have flat-rate bundles, probably similar to what you have now. Comparing them depends on your usage pattern, so again that's the first thing to consider, then you know what to shop for. If you need features like voicemail or voicemail transcription, be sure to look at whether you need an expensive bundle with it in, or whether you're better off paying for that seperately.
If your outbound calls are to a limited number of numbers, such as relatives far away or internationally, consider getting a similar VOIP system for those relatives. Most VOIP companies have free "on network" calls between their customers, regardless of the country they are in. So your most common, and most lengthy calls, could be free.
The Obitalk boxes (ATA's: analog telephone adapters) have an advantage here, if you install them in yours and relatives houses. As well as allowing you to use any of the "bring your own device" VOIP companies like those listed above, they have their own Obitalk network allowing free calls between their boxes, and also to/from their iOS and Android apps. There are other ATA's from other companies (Cisco have well-known models), and other ways to make free calls between them, so Obitalk isn't the only option.
I mentioned above I pay for the incoming number. Not every supplier has incoming numbers available in every area, you need to check this. Some can port-in (transfer in) your existing number, if you are attached to it, but not all can, so again check.
You can also get incoming numbers in other areas or countries, that ring on your home line (without forwarding costs). This means you can have a number near a cluster of relatives, who can call you with a local call. Doesn't directly save you money (each number has a monthly fee) but could save you having to call them back!