Me! I do, and I teach GCSE Maths as a resit subject, also in FE, so I have found a kindred spirit on Mumsnet.
Is this your first year of teaching GCSE in FE? You may find that the results for FE are nowhere near the national average for secondary.
There are some students who will make the effort, especially if they need it for university, but otherwise, you will find that most of them will have lost motivation and enthusiasm because they failed the first time round.
What does your HOD/ lead practitioner advise on how to motivate your learners?
Given how little time we have in FE for teaching a resit programme, I try to concentrate on the topics that they absolutely must know and are good at and leave out the grade 5 material.
The phone culture is very difficult to discipline and from experience, the rules are simply not consistent enough. Usually I tell them that I am giving them a minute to finish a phone update and then they put it in their bag or pocket. Try to incorporate using a phone in your lesson perhaps. In some lessons, I have got them to do a maths quiz on them or got them to use padlet to post contributions to a discussion.
If possible, try to get the program tutors involved, by adding them to your VLE groups so that they get the updates on homework and what they are doing in the lesson. I set plenty of work on MyMaths and the VLE and tutors know this. Sometimes when they are in tutorials with nothing to do, the tutor will suggest that they revise for maths and they are encouraged to log on to MyMaths or download something from the VLE.
Sorry for epic post. You're not alone in getting down when the students just don't care. You have done what you can to help them pass and in the end they will get the grade that they work for.