Advice requested on Outsourced Sales for scale

Hi folks,
Hope your efforts are bearing fruit. I'd love to get your thoughts on outsourcing sales (tips, providers, watch-outs etc.) in order to scale the company. For context, our company provides tech-enabled energy analyses to property managers, developers, and government entities in affordable housing. 95+% of work is inbound or renewal/subscription style revenue. It doesn't make sense at the moment economically to build out a full fledged Sales team in-house, but now that operational improvements and expansion inside existing customers have peaked, going after new clients is clearly the next step.

We have a good database of leads (numbers, contact info etc.), but need the manpower to email/call/convert. Any suggestions on an outsourced sales approach? Provider names, past successful stories, etc. are highly appreciated.

-Atul