"It's needing "object constancy" -- Quote:
Lack of Object Constancy: When a person is lonely, most of us can soothe ourselves by remembering the love that others have for us. This ability is known as object constancy. Some people with BPD, however, find it difficult to evoke an image of a loved one to soothe them when they feel upset or anxious. If that person is not physically present, they don’t exist on an emotional level."

Yes, that's the one. It goes back to early childhood experiences of separation and learning how to deal with it. For a young baby when the mother is gone, she vanishes altogether and ceases to exist -- they don't yet have a concept of understanding that the mother will come back

At this stage it could help if you ask for a transitional object (phone message, picture, an object T can give you) to hold on to.