Sorry if this is stating the obvious but both parties need to have their 'Presence' set to on i.e. Green.

So the caller sees his own Green presence symbol (next to Bluetooth in the status indicator area) and also sees in Contacts that the recipient has their presence status shown as green also.

Otherwise little happens.

Needless to say inconsistencies and anomalies crop up too but those are the basic requirements.

Please disregard if this is already known. Post back if I can help as I have a fair bit of experience with this now.