When can I mass text my list?

[John Horner]

If someone calls me from my website or a postcard about selling their house, but we can't make a deal at that time, is it legal for me to send them a text at a later date from a mass text service?

Is them calling me enough to be considered "opt in", or do I actually have to ask them on the phone or on a webform if they agree to text messages?

If I was personally texting them it wouldn't be an issue, but the mass text comes from a 5 digit number and is obvious it is a marketing text.