1. Visit
https://support.fongo.com/hc/en-us/arti ... ngo-Mobile, and go through the Android specific solutions.
2. Try disabling doze mode for Fongo Mobile. Here is an example:
https://eatsleepride-support.groovehq.c ... arshmallow.
3. Are you using a OnePlus device? Visit
https://support.fongo.com/hc/en-us/arti ... us-devices.
4. I doubt this is relevant since you mentioned "sometimes", but if you have a Samsung smartphone, visit
https://support.fongo.com/hc/en-us/arti ... s-Android-5. If this problem only occurs while you're on Wi-Fi at a specific location and if incoming calls never ring there, check to see whether you're dealing with a SIP ALG problem in your router (disable SIP ALG). Refer to points 1 and 2 from
viewtopic.php?f=8&t=20199. Also read point B from
viewtopic.php?f=8&t=20199#p78976.
Alternatively, follow the "Wi-Fi Router is blocking VOIP Traffic" section from
https://support.fongo.com/hc/en-us/arti ... ngo-Mobile. Port forwarding is a security risk. Don't port forward unless nothing else works.
6. If the problem happens intermittently while on Wi-Fi at a specific location, when there are bunch of other apps and devices fighting for available bandwidth, visit
viewtopic.php?f=8&t=20199#p78976. Read Point C. Substitute "ATA" with "smartphone" in the description.
7. If you're not on Wi-Fi, ensure you're in a good cellular signal area while testing.
If none of these suggestions help or if no one one else offers further advice, submit a ticket:
https://support.fongo.com/hc/en-us/requests/new. Provide them with the brand and model number of the smartphone being used, and, if possible, its firmware version.