[shakna]

DTMF [0] which also points to SDP, along with the tel scheme RFC [2]. It's a bit of a rabbithole to find what exactly is supported.

[0] https://www.rfc-editor.org/rfc/rfc4733#section-2.4.1

[1] https://www.rfc-editor.org/rfc/rfc3555#section-3

[2] https://www.rfc-editor.org/rfc/rfc3966#section-5.1.3