In our mobile operator there is no difference between the fixed and mobile rates. They are the same. MS to MS is around 80% while the rest is the others. In one house you may find one fixed and every family member may have a mobile and every family member is trying to call the other, one is at school , another one at work, ..etc. unless the mobile is unreachable he/she will not try to call the fixed line at home later.
Abdel