hi ferniegirl
good idea to have a general chat with the manager - keep it light touch - I find the folk in dad's care home are responsive and listen to me because I chat with them whenever I am visiting eg I always say I am there as a courtesy and ask how things are, and they know I am supportive and appreciative of what they do for dad - that way they know if I mention anything I'm not complaining as such, I just really want an answer or some help, and they are happy to act on what I say
I absolutely agree that it is unprofessional behaviour for any carer to have their mobile out when on duty - it's only ever happened in dad's home a couple of times and both times the carer was apologetic and explained that they were unfortunately waiting for some specific family news; neither time was the carer constantly checking the mobile - so mention it, maybe as a question about the home's policy or as a way of praising the other staff ... rather than an outright criticism - then if it ever happens again you can complain for real
it may be that the staff have some idea of when you visit, maybe you go at a regular time, so they know you will be around to support your mum - I know the carers in dad's home have busy sessions but every visit I see some thoughtful and particular connection with the residents, and they clearly know all the residents well
best wishes