An online service, Jaduka.com, will let you create an eCard postcard that will also include a click-to-call button set up on their service. Visualize the email recipient looking at the photo(s), thinking that this is a listing of interest. Or, perhaps they just have a question about your listing services because they're impressed and want to test this button. They click on the button, enter their name and phone number, and they're connected directly to you on their regular phone.
Keep in mind that they're in front of their computer, with your marketing email in front of them. If there's any inclination on their part to talk to you, this is the time. And, this is when you want to talk to them as well. There are other Click-to-Call services, and some of their buttons can be included in emails. However, this is the only one I've found that offers an integrated service. At this time, the eCards service is free, and the first 60 minutes of phone time on calls is free as well. It's pretty inexpensive after that.

