There's nothing wrong with the Cisco stuff, it's the access costs and call charges that kill you. The solution to high access costs is to cancel the ISDN and use SIP trunks which - in our case anyway are free. To do this you'd need two ordinary exchange lines and 2 broadband connections.
Sadly, I don't know how you would connect that Cisco PBX to a SIP trunk and suspect that it's not possible (one of our geeks will be able to tell me tomorrow). It may be possible bi-pass the in-house box altogether and use SIP directly to the Cisco phones but again that's fraught with problems because, although it's theoretically possible to force Cisco phones to accept SIP, in practice it often turns into a nightmare of installation and support.
From our point of view, we'd say look at selling the Cisco and going to either a hosted SIP solution or an in house PBX with SIP trunks. (For this last option you need to find a really good SIP expert - not Joe round the corner who thinks he knows something about VoIP) and you'd still likely need new phones
If you want to talk it through with someone, give us a call on 020 7043 5555, tomorrow.