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Not much info to go on, but :

You can back up your WhatsApp messages before the switch to Smarty just in case, although WhatsApp normally backs up to a cloud service anyway - check in WhatsApp settings, or search for videos on this on YouTube if unsure. 

If you are keeping the same phone, texts etc normally stay on your phone.

Malcolm_1310990  If you normally have your number associated with an app like WhatsApp, then you should not lose anything.

The information is stored on your phone and not usually on a sim. It's always a good idea to back up your phone data through one of the cloud-based systems on offer.