My father has an Eircom business line, single line like yourself, and he has call forwarding function, so it should be available to you. Not sure if there is a small standing charge for it, I think he is just paying for each call as it is forwarded to the mobile. The forwarding is done by typing in a code and then the number to forward calls to, and stats in place until you take it off. He has been using this for years and years, has the calls transferred to his mobile for 4 days of every week, so I'm 100% certain this works.
Get onto eircom again, this should be available to you, and I can find out more if you need me to