Do grandparents or greatgrandparents have any visitation rights. The mother will not answer our calls and will not let us see her.

My grandson has primary custody but has let the mother keep her longer the last several months while he has been out of state. Their agreement was that we [grand & great grand parents] would be able to get her 1st,3rd & 5th weekend of the month & every other holiday. And take her out of state to see her dad. [my grandson] It has been over three months since we have seen her & nearly two months since we have talked to her.When we did talk to her she would cry for us to come pick her up. Now the mother says the father is the only one that can pick her up and she will answer my grandsons calls but not ours. We have always been a very active part of her life for the first four year of her life we saw her every day and she is five now. we live on a very limited income please help us. We live in Dallas Texas.