A:
Here is what VTECH indicates:
VTech
Subject
Product Support
We regret to inform you that the CS6709 and the CS6419 will not work together because they belong to two different model series.
The compatible accessory handset for the CS6419 phone system is the CS6409 while the compatible main base for the CS6709 handset are CS6719/CS6729/CS6758/CS6759/CS6829/CS6859 phone system.
You can find the registration instructions in the Appendix section of the user's manual. To download a PDF copy of the manual, please click on the link below:
https://www.vtechphones.com/support/technical-support