Patients are paying up to £1.60 a minute to call their GP.
Twenty-five practices out of more than 200 across Sussex are still using 084 telephone numbers instead of standard geographical ones.
The cost of using them is the same as a local call for people using a landline but charges can escalate for those using mobile phones.
Practices have a system in place to ring people back to cut down on the cost of a call but patients say this does not always happen.
Patients using the Park Surgery in Horsham said they had ended up paying out £1.60 a minute because they used their mobile.
Another was charged £1.36 for 17 minutes and more than £1 of this was for when they were left on on hold.
The Department of Health has advised the NHS to avoid using 084 numbers if patients end up paying more to use them.
Surgeries using these numbers have been told to ensure a call back system is available.
A spokesman for NHS Sussex said: “None of our practices are in breach of their contracts by having an 084 telephone number.
“We are assured that all our practices which use an 084 number offer a call-back system. We regularly audit this.
“We have made practices aware of alternative telephone systems to the 0844 number and continue to encourage them to consider these alternatives as their contract term ends.
“We take our responsibility on this matter very seriously and are currently working with our practices to review what other steps, over and above the contractual requirements, can be taken to ensure there are no barriers to patients contacting their GP practice.”
A statement issued by Park Surgery said: “We are currently looking into establishing a local number for patients to contact us on.
“While we are advised by BT that our 084 number costs around 2p less per minute than the local rate for the vast majority of telephone contracts, establishing a local number alongside this will ensure that patients have the option to use an alternative number should they choose to do so.
“If a patient has a specific issue and needs to be called back we would expect our staff to be considerate to their needs.”
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