How often does your MIL call?
DP (of 20 yrs) mother is on the phone every night. I don't get involved personally as don't like the intrusion and needy/possessive/controlling aspects of her behaviour, but as soon as she rings he jumps up and practically runs into other rooms. Always has. I've mentioned it in the past and he's stopped it for a time but always reverts to this. And I'm not really into dictating or controlling others' behaviour. She's like a third person in the relationship (or more like first or second, me being the third!) So over it.