Perhaps this will help you find the path?

;WITH Transcript (message) AS (

SELECT '20:32:12 [Rachel] Thank you for contacting Our Company. How may I assist you?'

UNION ALL SELECT '20:32:31 [Marina Galofaro] What is the mailing address to mail a check?'

UNION ALL SELECT '20:32:49 [Rachel] I''m happy to provide you with the mailing address.'

UNION ALL SELECT '20:33:22 [Marina Galofaro] O kay, Thank you.'

UNION ALL SELECT '20:33:28 [Rachel] One moment please while I provide a link with directions to mail your payment.'

UNION ALL SELECT '20:34:05 [Rachel] Please click <a href="http://company.custhelp.com/app/answers/detail/a_id/1050/kw/payment" target="_blank">here</a> to view how to mail a payment.'

UNION ALL SELECT '20:35:07 [Rachel] Is there anything else I can help with, Marina? '

UNION ALL SELECT '20:35:56 [Marina Galofaro] No, Thank you!'

UNION ALL SELECT '20:36:00 [Marina Galofaro] Session Disconnected'),

Operators (username) AS (

SELECT 'Rachel')

SELECT message, username

,operator=LEFT(operator, CHARINDEX(']', operator)-1)

,CHARINDEX(']', operator)

,TextOfMessage=LTRIM(RIGHT(operator, LEN(operator)-CHARINDEX(']', operator)))

FROM Transcript

CROSS APPLY (

SELECT SUBSTRING(message, CHARINDEX('[', message)+1, LEN(message))

) a(operator)

LEFT JOIN Operators ON LEFT(operator, CHARINDEX(']', operator)-1)=username

WHERE username IS NULL