I've been approached myself by similar companies in the past, but have always resisted their sales pitches. I'm sure that if I was to use them it would work, however I've always stuck to the traditional methods of networking and advertising to build my business.
I think you need to keep in mind that the telesales company will not promise you any new clients. What they will try and do is arrange meetings with prospective clients i.e. businesses who may wish to find out more information about your services. There is no guarantee they will sign up with you. Fees vary between different agencies, but expect to pay around £50 per meeting. Therefore before agreeing to meet a prospect, ensure you have adequately done your research e.g. are they the type of client you would be able to service, where are they located, what reporting requirements do they have etc?? The telesales company should really do this for you, but its worth checking yourself, after all £50 leads can soon mount up if they dont prove worthwhile!
I used a telemarketing company when I first set up, and it really did help me to attract the number of clients I needed to enable me to go full-time. I took the view that if I spent £50 to attract a client who's annual fee income was say £1K, then it was money well spent. You do obviously need to use other marketing methods, but when used as part of a well planned out marketing mix, I believe telesales can play a constructive part.