Validation of telephone numbers by third parties

[PanderMusubi]

Like #7 also telephone numbers can be validated against known formats.

Additionally, and only with full consent of the user (as telephone numbers are send to a third party), telephone numbers can also be checked by services such as:

• https://veriphone.io
• https://numverify.com
• https://cognalys.com

Within their free plans or after providing credentials. This would filter out phone numbers that are no longer in use.

[DDvO]

Sending telephone numbers (and potentially other personal data) to third parties for checking for would not only mean a major code extension but likely is not possible due to security restrictions on Thunderbird add-on. Moreover it would be a major data protection / privacy issue, and would on many countries not only require full consent by the add-on user but also consent of the data owners, i.e., the persons listed in the address book(s).
Therefore I certainly won't support that.

[DDvO]

See also my more general comment to #7, which also covers the first comment you gave here.
So I've restricted this issue to the more specific topic of third-party online phone number checking.