I think if the company allows the client enter into contractual obligation by phone it should also be possible to invoke the cancellation part of the contract the same way. These days we get into so many contractual obligations (I can't even count the number of different subscription services I use) exactly because it is easy to make them and the law should protect users by preventing companies from making it hard to cancel it.

So, if a company allows the client to create a contract by phone or Internet, it should also be easy to cancel it the same way.