Have you tried sending it to the direct inbound email address configured on it via the send email option in the alert?
I use Messagebird, much easier. Literally took 30 mins to setup and test.
Thanks for the replies.
I now know the answer to this so thought I'd provide an update just in case anyone else comes across this issue.
I actually got this working by un-installing and re-installing NPP, I then opened the application by right clicking on the desktop icon and selected 'run as administrator'. At this point SMS messages started to come through which was great!
However when the next on-call person needed to be moved over, they logged on to the server to make the change but the NPP application was not open on the screen so he double clicked the desktop icon which creates a second instance of the processes but also at this point the user profile (not administrator) is a virtualised instance which does not work for the reasons outlined below, I've had great support from NotePager Pro and the guy who dealt with the issue gave me this explanation:-
Windows Server 2008, 2012 and 2016, as well as Windows 7, 8 and 10, virtualize each Windows user profile. So, what you think of as "HKEY_LOCAL_MACHINE" in the registry is really a user profile virtualized instance of what's contained in HKEY_LOCAL_MACHINE that's actually nested in the HKEY_USERS section of the registry. It's only administrators and programs that are allowed UAC authorization that get to access the 'real' HKEY_LOCAL_MACHINE - and Solarwinds absolutely requires everything to function out of the real registry location.
This can be resolved by creating a 'localdb' setting that will force all user profiles to reference the same local data, a reboot of the server is then required so that Solarwinds can read the new registry variables. However, this doesn't always work, we are considering moving to PageGate as this can run as a Windows Service which means we could reboot the server and not have to start anything manually which is what we need.