While I do not have any more suggestions to help you find an SMS option for FPW, I can say that as a consultant occassionally assisting a few clients with the old Foxpro applications, I can tell you (and you most likely already know yourself) that most of those clients yearn for a 21st century application - even if it is only the look and feel of a 21st century application where 'under the hood' it is essentially the same application.
While they greatly appreciate the stability of an application that they have been using for years, they want something which can utilize current technologies (such as the SMS you mention or other features). And they want something that 'feels' like it belongs in this century.
Additionally, as your client base sees more and more of new 21st century features in other applications, they will continue to want them in your application. So it might be SMS capability today, but tomorrow they may want something else.
Admittedly it is good that you are not using an old DOS version.
As I said above, the conversion from FPW2.6 to Visual Foxpro might be relatively easy since all of the FPW2.6 code runs under VFP9. Heck there is even a 'converter' within VFP that can be used - although my own personal experiences with it has been mixed.
The biggest changes would come from re-working your Screens into Forms, but most, if not all, of you Screen snippets would go directly into various VFP Form methods.
The other area of change, if you are not already done so now, would be converting your @SAY/GET reports to using Report Forms.
But once the conversion was done, you would then have a whole world of functional options that were not available to you under FPW.
Good Luck,
JRB-Bldr