Set your own Python script or executable as tel link handler

tel_links.py

# manage windows tel links
import sys
import os
import logging


log = logging.getLogger(__name__)


def init():
    """
    Set our dialer as tel link handler. Currently only for windows.
    """
    if sys.platform == "win32":
        import winreg

        if sys.argv[0][-3:] == ".py":  # we are not running with pyinstaller
            key_value = f"{sys.executable} {os.path.abspath(os.path.join(os.path.split(sys.argv[0])[0], 'dialer.py'))} %1"
        else:
            key_value = f"{os.path.join(os.path.split(sys.executable)[0], 'dialer.exe')} %1"

        log.debug(f"Register for 'tel:' links: {key_value}")

        key = winreg.CreateKey(winreg.HKEY_CURRENT_USER, "SOFTWARE\\Classes\\tel")
        winreg.SetValue(key, "", winreg.REG_SZ, "URL:tel")
        winreg.SetValueEx(key, "URL Protocol", 0, winreg.REG_SZ, "")

        winreg.SetValue(
            winreg.CreateKey(winreg.HKEY_CURRENT_USER, "SOFTWARE\\Classes\\CCAgent.tel\\Shell\\Open\\Command"),
            "", winreg.REG_SZ, key_value)

        key = winreg.CreateKey(winreg.HKEY_CURRENT_USER, "SOFTWARE\\CCAgent\\Capabilities")
        winreg.SetValueEx(key, "ApplicationName", 0, winreg.REG_SZ, "CCAgent")
        winreg.SetValueEx(key, "ApplicationDescription", 0, winreg.REG_SZ, "CCAgent Dialer")

        key = winreg.CreateKey(winreg.HKEY_CURRENT_USER, "SOFTWARE\\CCAgent\\Capabilities\\URLAssociations")
        winreg.SetValueEx(key, "tel", 0, winreg.REG_SZ, "CCAgent.tel")

        winreg.SetValueEx(
            winreg.CreateKey(winreg.HKEY_CURRENT_USER, "SOFTWARE\\RegisteredApplications"),
            "CCAgent", 0, winreg.REG_SZ, "Software\\CCAgent\\Capabilities")