Frequent travel can put stress on you and your family, and your employer should be concerned about it. So, if you're an employer, you really have to think about the personal side of what you're offering these folks. Reimburse employees for travel-related personal expenses. Just giving them a company credit card may not be enough.
Some firms even pay for pet care and lawn care for valued employees.
Let frequent travelers work from home as much as possible. Personalize travel perks. Offer concrete suggestions such as those given above to let them know you're serious about providing assistance and incentives. Focus on that phone call as you would an important business call. Call at a good time for them, preferably a consistent time they can plan around.