This page is just a reference of all the known Steam APIs, I do not know how they work. Please do not email me with questions.
UNDOCUMENTEDPOSTConfirmAddPhoneToAccount
https://api.steampowered.com/IPhoneService/ConfirmAddPhoneToAccount/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
fixed64 Nosteamid to use
string NoEncrypted token
UNDOCUMENTEDPOSTIsAccountWaitingForEmailConfirmation
https://api.steampowered.com/IPhoneService/IsAccountWaitingForEmailConfirmation/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
UNDOCUMENTEDPOSTSendPhoneVerificationCode
https://api.steampowered.com/IPhoneService/SendPhoneVerificationCode/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
uint32 No
UNDOCUMENTEDPOSTSetAccountPhoneNumber
https://api.steampowered.com/IPhoneService/SetAccountPhoneNumber/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
string Nonew phone number
string Nocountry code of origin
UNDOCUMENTEDPOSTVerifyAccountPhoneWithCode
https://api.steampowered.com/IPhoneService/VerifyAccountPhoneWithCode/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
string Nocode received from SMS