It's funny that you ask this because some of the cheaper SPA wireless phones natively support SMS via SIP but not the 7921/7925. With that said, they are very much geared towards medical use and there are vendors that enable SMS on these phones. I suggest you check out Radianta - you may need to actually contact a sales rep to see what product suite would be fitting for you, if any. You may also be able to come up with something via CUAE or a combination of CUAE and a third-party plug-in (Radianta has some of those as well - the Beacon suite). HTH.
Another option is Extension (www.opentheredbox.com). Text messages from PC to phone or group of phones and Phone to phone or group of phones. We also have the ability to send SMS text messages from Cisco phones to smart phones.