Yes, we will have to cancel the request on the advice of Mr. Tony, the Registrar Manager. ...(more below)
This is because he has already raised the request for a new telephone line for Jessie.
Sorry for the inconvenience.
1) will - indicates future tense to be performed later as a result of the advice.
2) on the advice of - information received.
3) Mr. Tony, the Registrar Manager - name the person before his title inside a sentence, to avoid brackets.
4) The reason - is not necessary before because.
5) because of - is not used before a noun.
6) by adding for the brackets can be removed.