Be persistent, but polite and friendly.
I'd go in person too, vs just a call if you are able to, if not a formal email/letter, or call and leave a message asking to speak to the hiring manager, etc.
Getting licensed to be a vet tech gives you a back up plan, but it is very different than being a veterinarian so if its not the career you want then look for other ways to get experience.
Everyone gave very good advice. Dont give up!