They could be trying any number of things, most likely finding a way to steal innocent people's money.
The one-ring scam (Google that for more info) is one way to get live phone numbers for them or other scammers that they sell your number to. The unsuspecting "phish" gets a one-ring call, and can't overcome the curiosity (which, you'll recall, can kill a cat) and they call the number back. BINGO! Here's a live number they can use or sell.
Other scam calls actually will try to sell you something that doesn't exist, or get personal information from you or threaten you with a (fake) law suit, for example claiming you owe the IRS money for past unpaid taxes. Not true, but some people get hooked. They claim you owe, say, $5000, which will become a lot more if it goes to trial (which it won't) but real scam is they offer to cancel the suit if you pay a lesser amount RIGHT NOW, say, $1500. It sounds like a deal, so you follow their instructions to pay by Western Union (which can't be stopped or returned) or some other method that can't be stopped, or worse, you give up your credit card number to the foreign stranger and you're out a lot of money.
Those are only some ways people are being scammed by telephone these days. Email scams are mostly being beaten by spam filters, so the scammers have resorted to phone calls.
rec'vd call 6/2/15, 9:09am. let go to answering machine. No message. caller showed L7 Scvc Call.......couldn't abbreviate service correctly.....hmm....I smell India.
Received a call from this number at 10:49 AM June 2. It did not go to voice mail. I called the number and was given the opportunity to have my number removed from their caller list. Said it would take up to 72 hours.