> They are likely using an Apple device, so they can't use Signal as the default SMS/messenger app and have to remember to use it for the right contracts. Sign

> They are likely using an Apple device, so they can't use Signal as the default SMS/messenger app and have to remember to use it for the right contracts. Signal supports SMS on Android because Android has API for apps to actually interact with the underlying phone SMS capability (because Android is a platform customized by OEMs). Apple does not have public API to interact with the hardware to send/receive SMS, although there's no limitation on using SMS in other apps. There are two messaging limitations in the platform: 1. The sms: url scheme can only go to the built-in app. Since this does not let you pre-compose messages, I've actually never seen it used in production. 2. The MFMessageComposeViewController is used to overlay iMessage with your application to send a chat message. Generally I've only seen this used in lousy applications (admittedly including Twitter and various Google applications in that list) - the modern approach is to use the Share and Action sheet functionality, which presents first and third party apps via equal heuristics.