Hi Talulah
This is a copy from another site. It work's for me
After this, I did a restart with my Galaxy A3 2016
I had the same problem. Here's a fix:
1. Open 'Messages' (SMS app)
2. Go to 'Settings' in the top right (you need to scroll down to see 'Settings' from the options, as the defaul pop-up window isn't big enough to show all of them)
3. Select 'Default messaging app'
4. Select 'Rich Communications'
5. Uncheck the 'Rich Communications' box
This has stopped the 'Unfortunately IMS...' message appearing every few seconds on my S5.
Hope this works for you, too.