The fact that you made reference to her not speaking English made me wonder if it was London-centric, because I know from my vast work experience in this sector that outside of London the typical demographic for a domestic cleaner is white-British female.
There is no shortage of people wanting to do the job in London, but outside of it it's a different story. Rates for cleaning are a lot higher outside of inner-London, and customers are plentiful. This has everything to do with your OP, because if you're in an area where there's plenty of work then you have to be careful what you say, because like it or not, people who do this job will exercise their right to dispense with a customer if they choose to.
The £19ph is also relevant, seeing how you brought it up in your OP, but are later saying the lady comes from an agency. If she's being paid £19ph to turn up via an agency, where nothing but insurance is provided (as in, you give her all that she needs to clean with, and she isn't responsible for washing cloths & maintaining equipment etc.) then £19ph is a fair price, in fact bordering on the generous for agency rates.
If it's £19ph including agency fee, then for someone self-employed they'd need to be taking approximately £15.70 per hour just to be paid the same as minimum wage on a payroll, when other factors are added in, such as holiday pay. She will also not be paid for travel time as a self-employed worker from an agency. Therefore, if she's being paid a low hourly rate, I would certainly beg that you factor in some kindness when mentioning you don't like her being on the phone for the five hours she is.
And before you pull me up for saying self-employed, I am taking this from your use of the specific work "agency", which has a very specific meaning in the cleaning industry, and one which cannot be applied to either a cleaner who is self-employed & who finds their own customers, or a cleaner who is either employed or self-employed and works for a cleaning company.
No one working for an agency is going to be an employee, as that's not how cleaning agencies work.
Kindly, I have answered your question to me, would you now go back and answer mine? I asked if she did any cleaning work that was noisy, such as vacuuming, and if she was still talking on her phone whilst doing that.