There appears to be a known issue with the current firmware release (5.4.3) for Polycom Trio 8800’s where during a call, the keypad fails to appear meaning that you are unable to enter DTMF tones, or add additional callers.
Polycom have acknowledged the issue via a community support post which can be found here: http://community.polycom.com/t5/Skype-for-Business/Polycom-Trio-no-keypad-during-call/td-p/87244
UC software version 5.4.4.7776 (once released) should address this known issue. For now, the workaround is to remove the NTP details from the device, save and reboot, then re-enter the NTP details.