GE Concord 4 alarm with wireless gives Phone Failure

I call myself a pretty good handyman but have never worked on Alarm Systems. I see 'System OK' green light but after alarm goes off, I see 'Phone Failure' message. Confirmed with ADT and they also don't get any notification of alarm going off. They do get notified about the alarm being turned off using keypad code. I came to know as my alarm went off and was on for a while but I never got a call back.. so I did testing with ADT and they confirmed some issue exists.

There is a big panel which has all circuits and a battery. (A while back I had change battery message on myadt but I don't see it anymore).. There is also a small unit beside this big unit.. there are 4 lights on that small unit.. all are blinking.. 2 green and 1 yellow blink together and 1 red blink separately. I believe this small unit is for connecting wirelessly with central control.

ADT would like to send a technician but I would like to see if I can fix it myself first. Any suggestions would be appreciated.

Thanks.