However, the OP said that he did not want to carry multiple items. If his/her provider is a GSM carrier, I would recommend moving to the N900 as an initial solution.

Beyond that, it would be a process of utilizing VoIP and SIP services on your N810 to duplicate some cellular features, however, you'd be limited by whatever wi-fi access points you are connected to. This would limit your ability to chat-in-transit, but in some stationary points be usable for use.