I think it is potentially misleading to characterize the attachment issue as dependent vs independent. One being bad and one being good. Or one being too much and one being not enough. Feast or famine.

The goal is interdependence, and developing the "feel" of that requires consistency and constancy from a T. Contact between sessions on a "call me if you need me" basis can be problematic because it isn't constant--the conditions surrounding it vary, and that creates emotional confusion. Often this seems to evolve into an issue of boundaries, with "privileges" of limitless contact becoming restricted, and that action can activate feelings of abandonment.

The other issue with between session contact is that many Ts can't maintain consistency when contact is irregular, whether it's because of between session contact or because of differences in scheduled contact. Or the mix of the two. So the risk to a client is getting a response--or not--different from what was expected, and that can also activate feelings of abandonment.

Any of these situations can also increase anxiety about attachment which then gets expressed by increased emotional dysregulation. This doesn't mean that there can't be an arrangement about out of session contact for emergencies--but what constitutes an emergency needs to be spelled out clearly so that the client isn't left wondering whether they "should" or "shouldn't" call.