Since I updated my Lumia 920 to Lumia Amber, I discovered that sending SMS to multiple contacts has become impossible. Noticed that immediately after it was updated, my network carrier says it is a phone setting that has caused that. Can anyone help or have the same issues?
In the Messaging settings, at the bottom there is an option:
Group text (on/off) Keeps replies in one thread using MMS.
Is this option turned on? If it is, then the reason your group SMSes are failing is probably because your phone doesn't have MMS settings for your carrier.
Either turn this option off, or try going to Settings app > access point to configure MMS settings for your carrier.