Should I call over the internet?
No, you can keep using ISDN, GSM or even a combination of several solutions. This is all completely transparent to the user.
All these options are certainly an extra cost?
No, at 3CX nearly all options are included. Your only choice is the Standard or Pro version and the Hotel Module.
Is this a reliable solution?
This is actually a very reliable solution. Our more than 60 customers are the best proof of this after working with 3CX for years. Among our customers we even have several big hotels.