"my computer has a dial up connection, can i use my phone to connect to my computer via bluetooth and use this connection?"
i'm not sure if you can but why would you want to? dialup is rediculously slow and your phone data connection (through your cellular provider) should be much much faster. if you have your home set up as a wifi network then yes you could probably connect but for dial up it is not worth the trouble.
or did you meant to say:
"i want my phone to connect to a dial up connection and have a device use this connection over bluetooth"
your cell phone does not have a modem. it is impossible to do this.
it is possible to share your 3g/4g phone data connection with a device however, provided that they share either a physical data link (usb cable) or you make your phone a wifi hotspot. i'm aware of a way to do this on android phones but do not know how to set this up on other smartphones.