Can I use IP phones once I have terminated analogue lines into a VG224 from my UC500 paltform?
The VG224 provides 24 FXS ports which can be used to connect analog phones, fax machines etc. It cannot be used to connect external analogue lines.
You need a device with FXO ports for that purpose. You can fit up to 8 FXO ports into a UC520. If you need more trunks then it makes sense to look at an T1/E1 link or optionally SIP trunking.