To join three or more callers in the same conversation, a conference bridge is required. A conference bridge is a media resource that mixes multiple audio streams into a single stream, allowing multiple participants to communicate simultaneously.
A hardware-based conference bridge uses DSP (Digital Signal Processor) resources to mix audio streams. It is highly scalable and provides better performance compared to software-based solutions, making it ideal for handling multiple participants or high-quality calls.
Hardware conference bridges are typically required for real-time audio mixing in larger environments, as they offload the processing to dedicated DSPs on the gateway.
Reference: https://community.cisco.com/t5/collaboration-knowledge-base/basic-hardware-base- ios-media-resources-configuration-and/ta-p/3164231

https://community.cisco.com/t5/collaboration-voice-and-video/understanding-troubleshooting- mwi-on-unity-connection/ta-p/3162948

https://www.cisco.com/en/US/docs/voice_ip_comm/cucm/bat/9_0_1/CUCM_BK_C22BD805_00_ cucm-bulk-administration-guide_chapter_01.pdf

Software MTP - Software-only implementation that does not use a DSP resource for endpoints using the same codec and the same packetization time.
Hardware MTP - Hardware-only implementation that uses a DSP resource for endpoints using the same G.711 codec but a different packetization time. The repacketization requires a DSP resource so it cannot be done by software only. Cisco Unified Communications Manager also uses the term software MTP when referring to a hardware MTP.
https://www.cisco.com/c/en/us/td/docs/routers/access/vgd1t3/rel1_0/software/configuration/guide/ VGD_transcoding.html