Full creds go to jameson on Amazon :

1st,
CHOOSE A VERY HIGH QUALITY MOBILE PHONE - THE NEWER THE BETTER!
The latest versions have strong and robust BlueTooth signals, and that is the weakest link in this entire chain.
I have no affiliation with any manufacturer or supplier, but I would recommend iPhones, as they will do higher definition audio, especially when using Audio/FaceTime.

2nd.
As the instructions say, USE A VERY HIGH QUALITY, MINIMUM 2.1A 5V power supply that is NOT sharing its supply with anything else.
Also, I recommend replacing their short cables with more high quality ones that can be bought right here on Amazon!
USB male > USB MICRO (3-6 feet) depending on how far away your power supply is.
You will also require a USB > Mobile Phone Charging Cable (unique to each brand/model). This helps ensure the phone stays charged while in standby.
The Mobile Phone does NOT charge while you are using the Cell2Jack.

3rd.
Go to the www.cell2jack.com/setting.html website and check out the SETTINGS that are adjustable.
**REMEMBER, AFTER CHANGING EACH SETTING, YOU MUST POWER DOWN THE CELL2JACK AND RE-CONNECT TO THE PHONE**
NOTE: YOU CAN ONLY CHANGE **ONE** SETTING AT A TIME, SO HAVE SOME PATIENCE!!
You need to reboot Cell2Jack each and every time!!!

4th. MODE CHANGES:
Change the MODE from MONO to MUSIC by using the POTS Telephone and pressing 61#.
You will hear "an automated acknowledgement that Cell2Jack has disconnected" when it has completed it.
Reboot Cell2Jack.
IMPORTANT, for this MODE CHANGE, "Forget Cell2Go" on mobile bluetooth settings.
Reboot mobile device.
Reconnect to Cell2Jack by Pressing #*# on the POTS Telephone and "re-pair" Cell2Jack to your mobile phone.

5th.
Change the SPEAKER VOLUME to MAXIMUM by Pressing 28# on the POTS Telephone
You will hear "an automated acknowledgement that Cell2Jack has disconnected" when it has completed it.
Reboot Cell2Jack.
Reconnect Bluetooth on mobile device.

6th. Change OFF HOOK VOLTAGE to MAXIMUM by Pressing 74# on the POTS Telephone
You will hear "an automated acknowledgement that Cell2Jack has disconnected" when it has completed it.
Reboot Cell2Jack.
Reconnect Bluetooth on mobile device.

Now you should have almost as good a 'landline' at the Cell2Jack’s RJ11 jack as a regular wired landline could ever provide, if not better, with full duplex (talking and listening at the same time) capabilities, with the ability to transmit (SPEAK) and receive (HEAR) AT THE SAME TIME, INCLUDING MUSIC!!!