On I phone se / would like to increase the No. of ring’s the phone can handle.

That is controlled by your carrier, but not all of them can increase it. It is not determined by the number of rings, but by timing. From the time the call is created and passed over to your device. Generally it is about 30 seconds total. The carrier may be able to increase it slightly, but not much. See what they have to say.

How to change your ringtone:

1. Go to Settings > Sounds & Haptics.
2. Under Sounds and Vibrations Patterns, tap the sound that you want to change.
3. 
4. Tap a ringtone or alert tone to hear and set it as the new sound. 🔊