While it varies from one carrier to the next, you can often estimate the cost of two unlimited phone lines by reducing the cost of an individual unlimited plan by 20 percent and then multiplying the result by two. For example, if a plan costs $60 per month, you should reduce it by 20 percent (leaving you with $48) and then multiply that number by two. This would mean you could expect to pay around $96 for two unlimited plans (in this example).