You will setup call forwarding from your existing number to iPlum number and use Auto Attendant (IVR) feature of iPlum.
Yes, you can configure mobile or landline number with phone tree configuration.
If you stay with existing carrier, you can continue to do texting there. If you port your number into iPlum you can text using iPlum app.
For any questions, you can always contact us. We love to help our users.