An at-will contract is one in which the client is free to end the relationship at any time without penalty. This contrasts with a long-term contract. Leases, for example, are typically long-term contracts. A contract with a cleaning company, however, may be an at-will contract; the business contracting with the cleaning company is free to stop using cleaning company A and start using cleaning company B at any time. Yet tortious interference can still apply to at-will contracts.
Tortious interference is a fancy way of saying that one business deliberately, knowingly and with malicious intent interfered with the dealings of another. It is an age-old problem. In 1793, the English courts heard a case whereby the crew of a ship known as the Bannister, went ashore in Africa intending to trade with the locals. A rival ship called the Othello, looking to trade with the same group, promptly fired into the gathered crowds. Understandably, the locals scattered and refused to trade with the Bannister. The Bannister filed suit claiming tortious interference on the part of the Othello because they had knowingly and deliberately interfered with their ability to conduct business.
Today, tortious interference tends to occur when one company convinces another to break their contract with a third party by using devious means. A company may deliberately undercut another company's pricing to convince a client to break their long-term contract with a different company and come over to them.
Things get trickier when an at-will contract is involved. It would seem that if the client is free to break off the contract at any time, that any company could come to them and convince them to end the contract. That's not the case. While any company is free to approach them, and offer them the same deal they would offer any company in the same circumstances, they cannot engage in dealings specifically designed to undermine the first company in a desire to solely get the company to switch their carriers.
Obtaining and retaining customers is at the heart of any business. While legitimate competition is healthy, no business has the legal right to engage in underhanded dealings in order to steal customers from another company. If you believe that has occurred, you may wish to consult with a lawyer in order to understand how to take steps to protect your business.