Hello @kinza3,
If you active telephony service then is not voice register pool ; this command is SIP registering and you need voice register global instead of telephony services. Telephony service is for SCCP firmware. 8841 support only SIP Firmware.
Additionally, it's important to note that the Cisco ISR 2811 is an older router model, and it may have limitations in terms of supported phone models and firmware versions. You may need to consider upgrading your router or using a different CME-compatible device that supports the Cisco CP-8841 phones.
Best regards
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