Is this still the actual situation? In germany estos is very wide spread and used with lots of different telefone systems like Siemens, Elmeg, Snom and so on. And there are lots of features in estos that could not be found in the 3CX assistant. Even the look of the estos client, the central management and the integration of different databases and crm systems is great and all customers of estos we meet are more willing to buy a new telefon system that supports estos than changing to another cti client.
I think a server based tapi for 3CX to integrate estos would be very useful. If I'm not wrong there are even the first asterisk based systems that support estos. So a customer that is interested in changing over to a software based voip system may decide to go to this direction if he still wants to use his estos licenses. I would prefer if i could sell him a 3CX because i like the software very much

Just my two cents.