Stepsons don't want to visit their dad unless they are going on trips or receiving gifts?

he calls them during they week they always say "I will let you know" (if they are going to come over) then you never hear from them. he has been divorced for 4 years, separated for over 6 years. the oldest son has never forgiven him for divorcing his mother, now the 14 and the 17 year old are actually talking and hanging out together which this has never happened since they were little. The 14 year who used to come over regularly now tell's his sister i hope dad doesn't call me and want me to go over...... yet when there's a trip or birthdays/holidays they are definitely around. Should my husband keep calling all the time and begging them to see him? And pay for them to go on vacations with us when they don't see us any other time?