I'm having the same problem on my windows phone 8.1 , I can call the person but I can't text him is there a fix to this?
Welcome to the forum, Jeffrey.
However, you are bumping a thread that is over a year old and shows it was a carrier side glitch that got resolved. Unlikely that the solution then is the same as now.
You did say WP 8.1 do thanks for that. Can we get some more details?
What phone? It is on ATT? Do you have cellular data (internet)? Dual SIM by any chance?
Have you inspected the APN settings to make sure they haven't changed?
Use this site to find your country and carrier then use the values they provide:
World Wide Mobile Phone Settings Or check the help pages at your Carrier's website.
WP 8.1: Settings > cellular + SIM > SIM settings > toggle "Manual Internet APN" to ON > tap "edit MMS APN" then input the values into the fields and selecting "save"
Have you tried powering off > remove SIM and clean > reseat SIM and reboot?
Have you contacted the carrier?