If they're calling you claiming they have jobs ready for you but you just need to sign up to get them tell them where to stick it.
Generally, you don't need to pay for leads or business. And you shouldn't encourage it either. There's no need to have a middle man, there's you, the customer, and nobody else in between and that keeps profits decent and costs to the customer down.
You end up bidding for work, and paying for that too!
At least with most of these website you do.
I'm still yet to find one with more customers than tradesmen. The moment I do, they'll be sponsoring our several trade related websites.
Until then, you guys stick to all the free directories you can get yourself listed in. And if you have money to spend, about £300 - £500 initial setup and about £50 per year thereafter, get yourself a website and rank yourself in search engines!
I keep repeating myself saying all this. Been saying it for years. But I bet I'll keep saying it for years! They've just not changed these sites at all.