On the recent Microsoft 365 roadmap I noticed that DTMF support is coming to Microsoft Teams clients running in Citrix VDI. At the moment (June 2021) the feature is in development and its release is schedueld for this month, June 2021.
DTMF stands for dual-tone multi-frequency (DTMF) an is a telephony or calling capabiltity. You might use or know it, for instance, from using your (mobile) phone to join a conference calling a phone number and typing in a conference ID followed by a # to enter the conference. This capability is not yet there and required for some calling use cases. Therefore, Microsoft is working on the support for the Teams client in Citrix VDI, as the roadmap feature ID 83014 states.