Inserting a pause into a tel:// link

You can use 'p' to pause, for example, tel:12345678#123. By changing '#' to 'p' (or comma) will cause pause for extension.

tel:12345678#123  // original tel number

tel:12345678p123  // after changed for tel scheme
tel:12345678,123  // Use comma instead

Try using a comma instead of any letters. Back in the days of modem dialing, we used the comma to represent a pause but you may need to place two of them back to back to be sure the call is picked up before the extension is dialed. There is no way to tell your phone how to detect when to enter an extension, and menus may change so I would recommend just leaving the extension out anyway.