Is it permissible for an expatriate husband to engage in sexual activity with his wife over the phone (through words and imagination) to alleviate the intensity of sexual desire, or is that forbidden and feared to lead her into sin, and is it permissible for him to masturbate as an alternative?

There is no harm in a husband and wife speaking over the phone about intimate matters, as long as it does not lead to anyone overhearing them or to anything unlawful. However, if there is a fear of slipping into the unlawful, then it is obligatory to refrain from it as a preventive measure (sadd al-dhara'i'). Leaving it is preferable due to the unpredictable nature of its consequences.

It is recommended to strive for the wife to reside with her husband. If that becomes difficult, then he should fast, guard his hearing and sight, occupy his time with what benefits him in this world and the hereafter, associate with righteous people, and frequently supplicate and seek protection with Allah.

It is obligatory to provide an independent dwelling for the wife where she does not mix with non-mahram men.