What are the rights of brotherhood for the the sake of Allah, and are friends justified in reproaching the questioner for her infrequent phone contact?

Time is a great blessing, and many people are deprived in health and leisure. Communication between sisters is a good thing, but it should be in moderation and free from anything that displeases Allah, such as backbiting, slandering, and mocking. They do not have the right to blame you for lack of communication or not responding. There is no specific religious ruling for how much friends should talk, but the guiding principle is that a Muslim should not waste their time on what is not beneficial, or let it distract them from obligations or what is more beneficial.