It’s easy to assume that Mary’s Immediately telegram is from Mycroft because she says England needs her help (which can be interpreted as Mycroft aka the British government) and because we later see her at the Diogenes receiving instructions from Mycroft.
However, that scene at the Diogenes actually doesn’t take place until months later within the storyline of the Ricoletti case. Mary receives the telegram the day that John and Sherlock head to the morgue to check out Emelia’s body. The scene with her at the Diogenes isn’t until after Mycroft has convinced Sherlock to accept Lady Carmichael’s case, which John specifically tells us in his narration is several months later.
Just because Mary has been summoned there by Mycroft at this point in the story does not mean that the telegram at the beginning was from him.
