When reviewing my text messaging from my phone, I only see 5 numbers instead of the 10 digits for a phone number.

When reviewing my text messaging on my phone, there are only 5 digits displayed instead of the 10 digits for a phone number. 82539 and 98626. But then sometimes I see the full number. I think that the 5 digits are the other phone on my account but I don't know for sure.