To be able to send text messages out of Salesmate. You will need to enable the Salesmate Voice & Text app.
To do so follow the article: https://support.salesmate.io/hc/en-us/articles/360004733011-How-do-you-enable-the-calling-app-
You will have to choose a number which has the capability to allow text messaging.
Refer the capability column while purchasing the number to do so.