Found description format telephone-event
Capabilities: us - 0xc (ulaw|alaw), peer - audio=0x16 (gsm|ulaw|g726)/video=0x0 (nothing), combined - 0x4 (ulaw)
Non-codec capabilities: us - 0x1 (telephone-event), peer - 0x1 (telephone-event), combined - 0x1 (telephone-event)