There is a sort of way around this.

You can generate phone numbers with embedded DTMF codes - including pause and wait.

For example `<a href="tel:+447700900000,12#">` will pre-fill the dialler with that number. After dialling, it will pause for a few seconds then sent DTMF 1 2 #.

You can make it slightly interactive with the "wait" command. For example `<a href="tel:+447700900000;#9">`. The semicolon should* give the user a prompt asking them if they want to send the DTMF `#9`.

Of course, this is no use if the app is to be used interactively. But if you know that you need to send a predefined sequence, it is useful.