During my turn, my husband keeps messaging and calling my co-wife. Is that fair?

Necessary communication such as matters about the children or emergencies is allowed; but spending one wife's turn chatting affectionately with another undermines fairness and the feelings of the wife whose turn it is.
Allah commands justice between wives and forbids inclining wholly to one so that another is left suspended (an-Nisa' 4:129). Scholars explain that a husband does not enter another wife's home during one wife's turn except for a need, and the same spirit applies to today's calls and messages. Explain calmly how it affects you, and suggest a shared rule such as important calls only or set times. The other wife should also respect her co-wife's turn. If the unfairness continues, there are channels at the Syariah Court. See the question on visits during a co-wife's turn.