I think you have to go with your gut on this one. We've worked with clients to coordinate the use of telemarketing services through UpWork and have had a good experience. But we stuck with native-English speaking freelancers. Most on them were from the US.

When they call, ask them to not only explain the service, but also ask them to request an email address where they can send more information. Give the telemarketer a company email address and a pre-set message about your service to email. Or you can do this yourself. If you're simply trying to raise awareness, a phone call is no big deal and would be great. If you're trying to sell, it will be a little tough, and it might be better to simply have them setup a time for you to return the call and close the sale.

Whenever you're having to talk and explain a service over the phone, it's important to have a firm grasp on the language and vernacular phrases.

Look into some of the telemarketers from outside the US who list themselves as expert English speakers and setup a time to talk with them over the phone to gauge their phone skills and grasp of the language.