Unless you have protection against having to accept off duty calls under the terms of a union agreement or employment contract, as an "at will" worker your company can set the conditions of employment much as it sees fit. Accordingly, you must return these calls and if you don't, your employer is well within its legal right to discipline you (up to and including suspension or termination). That having been said, if you are paid hourly (versus beig paid  salary), then you have a right to be compensated for all times spent on these work calls.