sms difficulty to voxi?

follow up to my difficulties wiv sendin sms to voxi numbers,have tried sim in different handset on the advice of of one of your colleagues Rebekah (as it happens an iPhone) & receivin netwrk update from Smarty & confirmed in settings, messages that ability to send sms is enabled tried to send (numerous times actually) & says "not delivered" & "your message was not sent, tap try again to send this message" when click on red exclamation mark icon, any ideas.?