You have to have a digital/direct line assigned to your extension for someone to text you from an external text device. If you don't have a digital line assigned to your extension you will not be able to either send or receive text messages to your mobile app or softphone. You can either purchase an additional digital line/user or you can log in with the extension which has the digital line assigned to it, or you can assign a digital line (if it hasn't yet been assigned) to your extension. I don't know how many digital lines your company has or where they are assigned so I can't advise you further than this.
I forgot to mention above...the external customer would then text to the digital line number (with no extension). It is already assigned to an extension so that extension would receive the SMS text messages.