If you can prove they called you, and actually asked you about your products or services, you have 6 months of implied consent. Go ahead and send them as many follow ups as you want.
To prove they actually called you, you could:
1. Record all the phone calls
2. Make transcripts of all the phone calls
3. Avoid phone calls. Seriously. Phone calls are very difficult to track and to use as proof.
"...the recipient made an inquiry or application on any of the items above [goods, services, land or interest; business, investment or gaming opportunity] within the six month period immediately before the message was sent"