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why phone media flow is going through CUCM?

I have a setup like this:

Phone 1 .....<SCCP>........CUCM cluster 1.......<H225>.........Gatekeeper 1.......<H225>.......CUCM cluster 2 .....<SIP>......SIP gateway (CUBE)...........SIP conferencing

It is found that the Phone1 media flow is going like this:

Phone server in CUCM cluster 2......SiP gateway

I have a wireshark connect to Phone 1,  and I found the media flow is going through a call manager. Why and how come?

I guess it's because MTP, but I didn't enable "MTP requried" in the H225 trunk and SIP trunk.  In the device pool of the related trunk, no MRGL is configured.  Also G.711 uLaw is used in both cluster.

VIP Super Bronze

why phone media flow is going through CUCM?

MTP Required forces an MTP to be allocated for all calls. Not checking it guarantees nothing; CUCM may still invoke an MTP if needed, such as DTMF interop (e.g. H.245 alphanumeric to RFC2833), H.323 Fast Start or SIP Early Offer. It's more than just the audio codec.

As for the lack of an MRGL: resources not assigned to at least one MRG are available system-wide.

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