Here is some very old code I had (don't include brackets [] in anything):
B4X:
Sub MakeCall
Dim EventJob As HttpJob
Dim U As String
U = "https://api.twilio.com/2010-04-01/Accounts/[TwilioAccountID]"
EventJob.Initialize("MCall", Me)
EventJob.Username = "[TwilioAccountID]"
EventJob.Password = "[TwilioPassword]"
EventJob.PostString(U, "From=[+TwilioNumberwithCountryCodePrefix]&To=[+NumberToCallwithCountryCodePrefix]&Url=[aURLonYourServerthatTwiliowillPostCallStatusto]")
End Sub