Stating the obvious, but you have activated your sim card and bought a Smarty plan ? (I am assuming you are new to Smarty, apologies if that is not the case)
Recently saw a guy complain on twitter that he could not get his newly acquired Voxi sim to work ...He had just put the sim card in his phone, he hadnt activated it and bought a plan....
You did check the quality of coverage where you live, work, etc., which is very important ?
Also are you using the SIM in a phone or router, are you in the UK, or in the EU or further afield, expecting to use roaming ?
What phone are you using ? IF its an old phone, it may be 2G only, and not compatible with the 3G, 4G and 5G networks used by Smarty.
