Replace Messages app with something else and have it use my phone number?
Solution 1:
You can turn iMessage off in the Settings app under Messages. Then your iPhone will only send SMS and MMS messages using your phone number.
You will also need to turn off FaceTime.
You will also want to make sure every iOS device you have has iMessage turned off, or you will just get iMessages on the devices that have it turned on.
If those steps don't work Apple provides a form to deregister your phone number from iMessage. It is nominally for Android users so I'm not sure if it will work for iPhone users.
You will not find a way to replace Messages as the default app without jailbreaking.
Solution 2:
According to a third party wanting the same, Apple doesn't allow this: https://support.signal.org/hc/en-us/articles/360007321171-Can-I-send-SMS-MMS-with-Signal-
There was once an app which synced your sms and allowed sending and receiving through an app: mysms. It would still work if you put the sim card into an android phone, but that doesn't seem to be what you want, as you'd now need another phone number for calls on your iphone...
The closest you can get on an iphone is probably through your provider's sms website offerings, like this one: https://n.vodafone.ie/business/products-and-solutions/mobile-communications/bulk-texts.html
Depending on your provider, you could probably put a shortcut somewhere on the iphone which opens a page where you can see your sms inbox and write new ones - with some business features on top.
Then you just need to deregister your phone number from imessage and make it work with your email address only. Anyone entering your phone number on imessage should then get to send the message in green.
Solution 3:
The big question is will your carrier allow you to deliver voice to one device and forward the SMS to another service or SMS number you control as opposed to sending your SMS to your iPhone. They overwhelmingly insist on delivering SMS to the same cellular radio as the device registered for voice, but might be able to make that change for you.
Once you find that out, there are all manner of SMS services that run server side that will receive SMS and turn them into APNS notifications for any number of apps to consume.
You also might need to port your number to a MVNO / VOIP provider and then get a new cell number and forward voice to the old number.
Lots of avenues to split things, but almost all involve duplications of cell service unless your carrier will enable a split delivery from one number to two services.
At that point, you choose any app that pleases you and deregister iMessage from any or all of your SMS numbers so other iOS devices will send true SMS and not iMessage notifications over the APNS and your cellular data connection.
- Figure your carrier options or switch carriers
- Decide if you need two SMS or one works
- Pick your app for voice and your app for sms
- Deregister all SMS from iMessage