When I was having BB modem driver problems with AOL, they told me they could "temporarily" give me permission to access via dial-up so I could download driver updates.
From that I infer that (a) you can't normally use dial-up if you've switched to BB and (b) they can physically enable dial-up access if you ask them (and pay??). Have you asked? It may cost very little if you're a good customer ... and threaten to move elsewhere!
With AOL you can EITHER subscribe to BB with unlimited use of dial-up by paying more (typically £10 a month extra on Silver BB) OR you can use pay-as-you-go dial-up on a normal BB subscription but incur call charges.