Oprima is the imperative for “Call” – I think it literally means “push” (as in “oprima dos” for “To discuss your bill, press 2” or “Oprima 555-1234” for “Call 555-1234”.)
Gratuito means “free, at no charge” and is standard for usages like this.
Should you use an exclamation point, don’t forget that Spanish usage is to begin a sentence ending with one with an inverted one. My advice would be to omit it altogether.
The phone numbers are posted underneath our mailing address, so there’s no exhortation for the client to call – merely a simple listing. The Spanish translation already uses teléfono to indicate the standard number. I’m just trying to distinguish the local number from the toll-free.