There is no way currently to edit the alert notification subject the way you want. The subject is "[HQ] !!! - ResourceName AlertName". The number of "!" is determined by the alert severity. You can edit the resource name and the alert name which will have some impact on the subject line, but you would not be able to make it be just a phone number.
If you would like to take the plunge and modify the code yourself, it's really quite simple. The email is generated in org.hyperic.hq.bizapp.server.action.email.EmailAction. You just have to modify createSubject() and createText() methods to get the email alert to be formatted the way you want.
We do plan on enhancing the alerting for SMS and for customization in the future. However, at the present time, we do not have a release date for that.
If this is a big problem, you could achieve the functionality you need by using tools outside HQ. For instance, a small perl script could easily parse and reformat an alert email then send the modified alert notification to the SMS address. All that would be required is a little knowledge about perl and maybe procmail (assuming you are on Linux, of course).