The first concern was that long-term, 18-hour-a-day electrical stimulation of the cochlea would eventually destroy the neural tissue. This concern faded entirely as animal studies were done, [17] [18] [19] and as the person-years of stimulation accumulated and there were no reports of implant failure due to loss of the ability of the VIII nerve to be stimulated. Even though no one has used a cochlear implant for more than 25 years, if we are going to see VIII nerve damage it would be appearing by now. Beyond this, a number of temporal bones have been donated by long-term implant wearers to be studied after their death. No hint of damage traceable to electrical stimulation has appeared in these. [20] Thus, by now it should be clear that cochlear implants are a lifetime solution, even for young children.

I personally think the radio frequencies (FM signals) used in cochlear implants are weak since they are working on a very small and limited area of range.

FM assistive listening systems are safe with no concerns raised for them even though they use radio frequencies. It would seem that would apply for cochlear implants. The cochlear implants have been approved by the FDA for two brands so if there were a concern, it would have been addressed long before now.

The cell phones tend to use much stronger radio frequencies simply because of the sheer distance required.

Here's an explanation of FM systems:

F-M Systems can greatly improve listening conditions for the hearing impaired in the presence of background interference and long distance from the sound source. F-M is an acronym for frequency modulated. F-M Systems can be used alone or with certain types of hearing aids. The speaker's voice is picked up by a microphone attached to a transmitter that clips onto the clothing of the hearing impaired. The transmitter converts sounds into a radio signal. The speaker also wears a transmitter around his or her waist, neck or in a pocket. The hearing impaired person also wears the receiver connected by a cord to a hearing aid. The receiver converts the radio signal sent from the speaker's transmitter into electrical energy that is sent back to the hearing aid for amplification. The F-M receiver enables the user to leave the immediate area of the signal at a distance of up to 300 feet and still receive sound clearly. For more information about F-M receivers, contact an audiologist.
(From HealthAnswers)

You can find out more about radio frequency safety at the FCC at this link:

http://www.fcc.gov/oet/rfsafety/

You can also see this comparison chart of various levels of radio frequencies of brands of cell phones at this link:

http://www.cnet.com/wireless/0-1923401-7-2587641.html?tag=st.wr.1923401-7-2587640.txt.1923401-7-2587641

Hope this helps.

http://www.cnet.com/wireless/0-1923401-7-2587640.html

By comparison, cell phones are over 10 MHZ and much higher (in the last group).

Therefore, the signals for the cochlear implant are much weaker.

The latest research into health concerns of cell phone radiation seems to indicate that they don't cause cancer but investigation is still continuing.

For comparison, you can see this quote below from the link listed at the end of it. It gives the ranges of frequency levels of different groups of devices.

(begin quote)
The International EMF Project was established by WHO in 1996. For the Project, EMFs are defined as electromagnetic fields with frequencies from 0 to 300 GHz.

Static fields (0 Hz): Magnetic levitation trains for public transportation, magnetic resonance imaging devices used in medicine, and electrolytic devices using direct electric currents for materials processing in industry.

Extremely low frequency (ELF) fields (>0 to 300 Hz): Trains for public transport (16 2/3 to 50 or 60 Hz, plus harmonics), any device involved in the generation, distribution or use of electric power (normally 50 or 60 Hz).

Intermediate frequency (IF) fields (>300 Hz to 10 MHz): Anti-theft and security devices, induction heaters and video display units.

Radiofrequency (RF) fields (>10 MHz to 300 GHz): Mobile telephones or telecommunications transmitters, radars, video display units and diathermy units.

http://www.who.int/peh-emf/

Note: Check also this link for a diagram on the page showing where different things are for various levels of frequencies:
http://www.mcw.edu/gcrc/cop/cell-phone-health-FAQ/toc.html

There seems very little to be concerned about regarding cochlear implant transmission frequencies since they are very low.