There is a Messaging plan that includes unimited texting and internet for $34.95. It is only available on 3GMM and "Feature" phones. On this plan, voice minutes are $0.45 each.
The smartphone version of this plan is $54.95/month, but it is only for users who are deaf or hard of hearing, and you need to provide proof of such before they will authorize the plan.
Do you actually talk only 20 minutes each month, or are you also making in-network calls to other Verizon Wireless users and/or night/weekend calls that don't count against your allowance? If you are making a substantial amount of "free" calls, the per minute cost on the messaging plan may be too high to justify the switch.