You'll need to pull SDL traces from CUCM to look at the individual outbound call legs being placed and the event timestamps. If the cell phone call leg connects at a signaling layer (e.g., a SIP 200 OK) between 1.5 and 19.0 seconds after CUCM initiates the call, the feature is working as designed. With Timer Control CUCM can't really tell if a human answered; it uses the timers to make an educated guess. For example, if the cell phone is turned off and a call immediately rolls to voicemail the Answer too Soon timer is meant to disqualify that RD. It's the same idea with Answer too Late: it should be less than the cell phone carrier's Call Forward No Answer timer. These timers aren't perfect however: CUCM can't tell if a user manually declined a call, sending it to voicemail, while it is ringing. Also these days, anti-spam call apps on smartphones sometimes trip up the timers. If the call is connecting within the allowed timer window you can either try to adjust them or change the RD Voicemail Policy to User Control so CUCM requires a DTMF digit acknowledgement that human actually answered the cell phone (i.e. not voicemail).
PS- All of these same Timer Control risks apply to the MS Phone System RD as well. For example, if the user declines the call from the Teams app and it rolls to MS voicemail CUCM will see the call connect within the allowed timer window and stop ringing the cell phone.
