Missed a call then got "account already in use"

[nanyte]

This is a new one for me. I was too slow picking up the phone and it went to voicemail. I tried calling the person back, but kept getting an error message, something along the lines of 'account already in use' or something like that. When I finally got through, he said he just hung up from leaving me a voice mail. Seems leaving me a voicemail made may account 'active' so I couldn't make a call.

Is this normal?