Ring time and mobile devices
If you are forwarding your telephone number to a mobile number then you may experience ring delays. These are normally an issue related to the time that the mobile network takes to locate the handset across the mobile network via its local transmitter, and are part of the nature of mobile phone networks.
You will find that the time that it takes to connect to a mobile phone varies, depending on things like weather conditions, we infill this time with a “ring ring” tone, to avoid the gap that the caller would otherwise hear.
As an alternative you might want to consider using VoIP on your mobile device, as the data connection to the device tends to be more persistent, and VoIP carries the call over that data connection.