All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and messenger.
Apparently, there is nothing wrong with what you have mentioned regarding the use of the mobile phone of the job in calling land lines free of charge, as this does not cause any harm. This is something that is authorized and permitted according to custom. Ibn Qudaamah said: “The customary permission takes effect as actual permission.”
However, if your husband knows that his work does not allow using the mobile phone in purposes other than those related to the job, even if this does not involve any charges or harm, then he should abide by this rule; otherwise, he would be cheating. In fact, a worker is trusted with the tools that he receives to be used for work. Thus, he should keep the trust regarding this and avoid betrayal. Allaah says (what means): {O you who have believed, do not betray Allaah and the Messenger or betray your trusts while you know [the consequence].} [Quran 8:27]
Allaah Knows best.