according to this document, Cisco recommends IPv4 and IPv6 as the setting for the phone IP addressing mode. IPv6 Only is not recommended for production environments. Does anybody know, why using this mode should be avoided?
Thank you for supposition, but for IPv4 compatibility Cisco is using MTPs.
For two devices (such as phones) that support mismatched addressing modes, an IP addressing version incompatibility exists when a device with an IPv4 address wants to establish a RTP voice stream with a device with an IPv6 address. To resolve this IP addressing incompatibility for media, Unified CM dynamically inserts a media termination point (MTP) to convert the media stream from IPv4 to IPv6, and vice versa.
... so you lots of your internal traffic to route through MTPs? In some scenarios, that could require MTPs on many sites and complicate the MRGL/MRG config and troubleshooting...
Setting "MTP required" option in Trunk or GW configuration also leads to the waste of MTP resources and multiplexes compexity of troubleshooting, but this option is recommended one. Are there another causes for not using IPv6 only addressing mode? Will Cisco TAC support such installation?
MTP Required on gateways/trunks has never been recommended. It should only be used in very specific scenarios and can almost always be avoided.