Here's what I'm trying:
curl -X POST -d "Body=Hi there, this is a test message from cURL" -d 'From=+1aaabbbcccc' -d "To=+1zzzxxxyyyy" "https://api.twilio.com/2010-04-01/Accounts/$TWILIO_CONNECTED_SUBACCOUNT_SID/Messages" -u "$TWILIO_CONNECTED_MAIN_ACCOUNT_SID:$TWILIO_AUTH_TOKEN"
Where $TWILIO_CONNECTED_SUBACCOUNT_SID is a account SID with the phone number I want to send from. $TWILIO_CONNECTED_MAIN_ACCOUNT_SID is the account SID that the connect button sent back to me (has a name like "Subaccount for ConnectApp: ..."). $TWILIO_AUTH_TOKEN is my (not the connected account's) auth token.
I've tried various permutations of the 3 account SIDs but can't get any of them to work. How do I send an SMS on behalf of a connected account?
I got it to work if I purchase a new number for the $TWILIO_CONNECTED_MAIN_ACCOUNT_SID account via cURL. But I don't want to build a UI for purchasing numbers in my app, I want to let users just pick a number they already own on Twilio, from their main account. Is that possible?
Twilio developer evangelist here.
According to the docs, under permissions:
Charge account for usage
This allows your Connect App to perform actions that charge your user’s Twilio account such as make and receive phone calls, send and receive SMS messages, and buy phone numbers. Your Connect App will not have access to resources in the user’s parent account, like phone numbers. Instead, your Connect App must buy phone numbers on behalf of the user, using the Account SID passed to your Authorize URL.
Emphasis mine.
So, you cannot use a connect account to send messages from existing resources. You will need to buy a new number to send.
An alternative to using Connect here, that would give you access to the account's resources would be to have the user submit their Account SID and an API Key/Secret pair. While it's never ideal to share credentials, an API Key/Secret pair can be easily deactivated without affecting the rest of the account.