I only upgraded my iPhone 6 plus to iPhone x a week ago and have this problem since upgraded. My iPhone 6 plus never had this problem until it got upgraded.
What I did:
Reset network settings and nothing has changed.
Turn off data then reset my iPhone x and nothing has changed.
Replaced sim card and nothing has changed.
Erased all content and settings and nothing has changed.
I checked Message setting. MMS messaging and iMessage are on. Tried turn off them and nothing has changed.
iPhone X using ISO 11.2.6.
Is there other way to fix this problem?