If you start with expensive media, you'll have paid expensive media for every customer, even though some happy customers would have paid you just by receiving a text message, a medium 20 times cheaper than a call with an agent.

To bring things back to the point, let's look at a simple case that only compares two means of contact: SMS and call with an agent .
Let's say the SMS costs you $0.85 MXN and causes 30% of people to pay after the first call, 15% after the 2nd, and 5% after the 3rd, and the call with an agent costs $17 MXN and causes 50% of the people who answered to pay after the first call, 10% after the 2nd, and 5% after the third, we will have the following cases: