Weeding out by phone does prevent unnecessary time wasted on an interview. Don't forget that this is your chance to ask them questions too. Email back and forth can take time and especially if the person is texting with their phone. A phone call allows you to talk and get a feel for each other. You should ask them about the ages and sex of the child, the hours of care they need, what days as in fulltime or part time. Find out if the child has been in a care environment before as in why are they looking for care and also when they need the care to start.
If all that sounds good to you then you can ask them other questions like what kid of care of they looking for. All the while you are thinking about you offer and if it sounds like it is what the family is looking for. If it is then invite them for a visit.
Do not let the lady just take over the call and feel like you are in the hot seat which I am guessing is what is concerning you. You are not calling to defend your daycare just to offer your services.