It comes as patients across England submitted a record number of written complaints, with Labour saying it "inherited a broken NHS that has been failing patients and staff for too long".
The King's Fund health think tank said patient frustration with the NHS was rising and called on the Government to make improving communication with patients a central part of its coming reforms.
NHS England figures show there were 1,067 written complaints made against hospital and community health services at the NHS Surrey Heartlands Integrated Care Board in the year to March – a 13% decrease on the year before.
Of all the complaints made, 23% were fully upheld, while 39% were partially upheld.
The remainder were dismissed.
Louise Ansari, chief executive of Healthwatch England said the figures were unsurprising, with problems accessing GPs and dentists among the most frequent issues patients contact them about – but she warned the true scale of the problem may be even greater.
"There are major gaps in the figures with no national data showing what type of people are most likely to make a complaint, how long people wait for services to investigate their case, and how satisfied they are with the response they receive," she said.
"Patients appreciate the pressures health and care services are under. However, the public deserves a complaints system that is easy to navigate, timely, empathetic, and used by services as a genuine opportunity to listen and put things right to prevent poor care in the future."
An organisation independent from the NHS, Healthwatch is planning new national research examining who is most confident to formally complain, and what barriers put other patients off from raising concerns.
Across England, there were 241,922 written complaints made by or on behalf of patients last year, including those made against GP surgeries and dentists.
This was an increase of 5% on the year before and the highest figure since records began in 2016-17.
The most common reason for complaints was communication issues, followed by concerns around patient care and the "values and behaviours" of staff.
Dan Wellings, senior fellow at the King's Fund, said the figures "reflect the rising frustration that people have over many aspects of NHS care right now".
He said basic communication is lacking, such as receiving timely appointment letters and test results.
He added: "Public satisfaction with the NHS is at a record low, and it will be a long journey to recover it. Improving how the NHS communicates with those it serves will be a key part of that recovery.
"As the Government embarks on its health service reforms, this needs to be front and centre."
A Department of Health and Social Care spokesperson said: "We inherited a broken NHS that has been failing patients and staff for too long. We will listen to patient concerns and be honest about the challenges we face as we take key steps to reforming the health service.
"Our 10-Year Health Plan will deliver the radical change needed to get the NHS back on its feet and build a health service that is fit for the future."