SMS still sent from the old Twilio number when I purchase a new one and select that as the default outbound number.
Since we do not want to spam the leads with different Twilio phone numbers, the old leads in HL will be stuck with the old default Twilio phone number. Only new leads got added to HL will be getting SMS from the new default outbound number.
1. Delete and Re-create the contact
The new contacts will be using the new default Twilio number.
2. Remove the old default Twilio number
We could remove the old Twilio phone number. That way all SMS will be sent from the new Twilio number bought.
3. Assign the new default Twilio number to the user, assign the user to the contact and log in as the user to send an SMS
If you want to keep the old Twilio phone number, another suggestion would be to assign the new Twilio phone number to the user and log in as the user to send those SMS. You could also assign the user in the Workflow action. That way all automated SMS will be sent from the new Twilio number bought.
More info: Phone numbers for users / Assign Twilio Numbers to Users