Your comma idea is exactly the sort of thing I meant,

and in fact, I prefer it to the callback-slash-API approach

if it proves reliable.

On iPhone, a comma in a stored telephone number

means pause for two seconds and then continue dialling.

A semicolon means wait until you manually tell it to continue.

So if your Twilio Dese access number were 0114-123-4567,

John's contact could contain something conceptually like

0114-124-567...