I have struggles throughout school and so did one of my brothers. No matter how hard we tried we were just not good in certain subjects... if she is doing well in most of her other subjects that means she just is not that great in the subjects and will struggle.
With homework have a set time to do it... and when you or hubby can provide her full attention in helping her out if needed (ex; set up the kitchen or dinning room as homework central, you can clean up after supper or make a list of items are are running out of , paying bills, read a book which she does homework. You can easily stop what you are doing and help when she asks).
I was horrible in spelling, practiced every night and struggled (Mon-Thurs) then on Friday when it was spelling test time I did horrible usually got a C minus or D. The teacher said I needed to try harder, well I studied each night WITH my MOM (who is a teacher herself). So sometimes you a person is just not capable of doing better.... BUT that does not mean you stop studying or stop doing the homework, it just means you do your best get help and that is it.
During the summer get booklets/workbooks in all subjects to just keep the study/homework atmosphere up (for me I had a hard time going from free summer to schoolwork, would take me almost the whole school year to get my mind back into it SO my mom started the workbooks and that did help with being able to keep in a schoolwork mindset. We had an hour a day mon-fri where we did the workbooks).
With the phone you can simple say that until you think she is responsible enough both in school and around the house she can get a phone. This means doing the homework and studying in the subjects she struggles with EVERY night (SUN-THURS study/homework, Fri & Sat no homework/study in the evening). It is a good thing to earn the rights to something, it teaches responsibility, respect and good work ethic, but you get to set the limits, rules & goals. I did not get mine till I was in senior in highschool because my parents knew that it would take too much focus off of schoolwork, my youngest brother who always gets A-B's got one sophomore year because he did not need to work as hard to keep focused at school work.