Hi Silktech,
I think you didn't lose compatibility, the problem will be the next update, which will revert back your changes and you will have to do them again (but the data in the database will persists).
Besides that, one of the main features for the next version are custom fields in items, so you could add a custom field "phone number". Once we release the next version, (2.2), please contact me, so I could help you upgrading from 2.1 to 2.2 and not losing your already changes, so after that you could upgrade as usual to future versions (2.3, 2.4 ,...).
Thanks!!