Depends.
Is Mum on board and supportive of your attempts to reduce DSD anxiety? If so, then I think she could be a positive support of your approach via telephone.
However, if Mum is undermining your attempts to reassure your DSD, and is (inadvertantly) making things worse, then eliminating/reducing calls can prevent this.
My DSS Mum used to reassure him when he was here by telling him to "not to worry" - well, he wasn't worried until she suggested that there might be something to worry about! Talk about textbook - it was word for word the example used in the CAFCASS Handbook for Seprated Parents of what "not" to do when talking to your DC's while they are at the NRP home!
Mum also used to tell DSD to "make the best of it" when she was here.
Both DSC's hated the fact that their Mum contstantly reminded them on the phone how much she, grandma and the cat were missing them; they were only here for a long weekend at a time!
DSS used to get anxious if we didn't answer calls from his Mum (we now know that she was putting pressure on him), so DP did, for a while, prevent the repeated intrusive phone calls by unplugging the phone - and distracting the DSC on the infrequent occasions they asked to phone Mum - but alongside that, he ensured that he set time aside for them to call her at a neutral time (not near bedtime, for instance). It eventually culminated in DSS telling his mum that he didn't want her to call so much, and her accusing DP of coaching DSS to say that. DSS started to ignore his Mums calls after that.
We have also implemented a "no mobile devices upstairs" policy to prevent the late-night contact by text/emai/skype, which causes a lot of damage to DSD relationship with us (we've since been reunited and she has told us about a lot of things we didn't even realise were going on). DSS hasn't got a mobile yet, but he knows the rules that apply to my DD, and to DSD when she stays, so knows that the same will apply to him.