You are absolutely correct to monitor the salt in your diet since a side effect of estrogen replacement therapy is fluid retention. However, I suggest you clarify with your physician why he/she chose to restrict carbonated beverages from your diet. For example, diet cola contains approximately 30 mg of sodium in a twelve-ounce serving. Gingerale, which is one of the higher sodium containing beverages has just 75 mg. A piece of bread has 123 mg in one slice. These values are not high, considering an adult could consume 2400 mg of sodium in one day.
There has been a fallacy surrounding sodium in carbonated beverages possibly because it is called soda. However, the carbonation is created by carbon dioxide (carbon and oxygen), not sodium. By the way, phosphorus can increase the loss of calcium.